@@              Coverage Diff               @@
##           release/3.9.0    #1862   +/-   ##
==============================================
  Coverage          66.70%   66.70%           
==============================================
  Files                 88       88           
  Lines               7029     7029           
==============================================
  Hits                4689     4689           
  Misses              2340     2340           

Index: auto-sizes.php
===================================================================
--- auto-sizes.php	(revision 3238281)
+++ auto-sizes.php	(working copy)
@@ -15,10 +15,11 @@
  * @package auto-sizes
  */
 
-// Exit if accessed directly.
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
-	exit;
+	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 // Define the constant.
 if ( defined( 'IMAGE_AUTO_SIZES_VERSION' ) ) {
Index: hooks.php
===================================================================
--- hooks.php	(revision 3238281)
+++ hooks.php	(working copy)
@@ -6,9 +6,11 @@
  * @since 1.0.0
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Displays the HTML generator tag for the plugin.

Index: hooks.php
===================================================================
--- hooks.php	(revision 3238281)
+++ hooks.php	(working copy)
@@ -7,9 +7,11 @@
  * @since 1.0.0
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Add the dominant color metadata to the attachment.
@@ -63,16 +65,17 @@
 		$attr['data-has-transparency'] = $image_meta['has_transparency'] ? 'true' : 'false';
 
 		$class = $image_meta['has_transparency'] ? 'has-transparency' : 'not-transparent';
-		if ( empty( $attr['class'] ) ) {
+
+		if ( isset( $attr['class'] ) && is_string( $attr['class'] ) && '' !== $attr['class'] ) {
+			$attr['class'] .= ' ' . $class;
+		} else {
 			$attr['class'] = $class;
-		} else {
-			$attr['class'] .= ' ' . $class;
 		}
 	}
 
-	if ( ! empty( $image_meta['dominant_color'] ) ) {
+	if ( isset( $image_meta['dominant_color'] ) && is_string( $image_meta['dominant_color'] ) && '' !== $image_meta['dominant_color'] ) {
 		$attr['data-dominant-color'] = esc_attr( $image_meta['dominant_color'] );
-		$style_attribute             = empty( $attr['style'] ) ? '' : $attr['style'];
+		$style_attribute             = isset( $attr['style'] ) && is_string( $attr['style'] ) ? $attr['style'] : '';
 		$attr['style']               = '--dominant-color: #' . esc_attr( $image_meta['dominant_color'] ) . ';' . $style_attribute;
 	}
 
@@ -138,7 +141,7 @@
 		return $filtered_image;
 	}
 
-	if ( ! empty( $image_meta['dominant_color'] ) ) {
+	if ( isset( $image_meta['dominant_color'] ) && is_string( $image_meta['dominant_color'] ) && '' !== $image_meta['dominant_color'] ) {
 		$processor->set_attribute( 'data-dominant-color', $image_meta['dominant_color'] );
 
 		$style_attribute = '--dominant-color: #' . $image_meta['dominant_color'] . '; ';
Index: load.php
===================================================================
--- load.php	(revision 3238281)
+++ load.php	(working copy)
@@ -15,10 +15,11 @@
  * @package dominant-color-images
  */
 
-// Exit if accessed directly.
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
-	exit;
+	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 // Define required constants.
 if ( defined( 'DOMINANT_COLOR_IMAGES_VERSION' ) ) {

Index: class-embed-optimizer-tag-visitor.php
===================================================================
--- class-embed-optimizer-tag-visitor.php	(revision 3238281)
+++ class-embed-optimizer-tag-visitor.php	(working copy)
@@ -185,12 +185,21 @@
 
 			$style_rules = array();
 			foreach ( $minimums as $minimum ) {
-				$style_rules[] = sprintf(
-					'@media %s { #%s { min-height: %dpx; } }',
-					od_generate_media_query( $minimum['group']->get_minimum_viewport_width(), $minimum['group']->get_maximum_viewport_width() ),
+				$style_rule = sprintf(
+					'#%s { min-height: %dpx; }',
 					$element_id,
 					$minimum['height']
 				);
+
+				$media_feature = od_generate_media_query( $minimum['group']->get_minimum_viewport_width(), $minimum['group']->get_maximum_viewport_width() );
+				if ( null !== $media_feature ) {
+					$style_rule = sprintf(
+						'@media %s { %s }',
+						$media_feature,
+						$style_rule
+					);
+				}
+				$style_rules[] = $style_rule;
 			}
 
 			$processor->append_head_html( sprintf( "<style>\n%s\n</style>\n", join( "\n", $style_rules ) ) );
Index: load.php
===================================================================
--- load.php	(revision 3238281)
+++ load.php	(working copy)
@@ -5,7 +5,7 @@
  * Description: Optimizes the performance of embeds through lazy-loading, preconnecting, and reserving space to reduce layout shifts.
  * Requires at least: 6.6
  * Requires PHP: 7.2
- * Version: 0.4.1
+ * Version: 1.0.0-beta1
  * Author: WordPress Performance Team
  * Author URI: https://make.wordpress.org/performance/
  * License: GPLv2 or later
@@ -71,7 +71,7 @@
 	}
 )(
 	'embed_optimizer_pending_plugin',
-	'0.4.1',
+	'1.0.0-beta1',
 	static function ( string $version ): void {
 		if ( defined( 'EMBED_OPTIMIZER_VERSION' ) ) {
 			return;
Index: readme.txt
===================================================================
--- readme.txt	(revision 3238281)
+++ readme.txt	(working copy)
@@ -2,7 +2,7 @@
 
 Contributors: wordpressdotorg
 Tested up to: 6.7
-Stable tag:   0.4.1
+Stable tag:   1.0.0-beta1
 License:      GPLv2 or later
 License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 Tags:         performance, embeds
@@ -67,6 +67,13 @@
 
 == Changelog ==
 
+= 1.0.0-beta1 =
+
+**Enhancements**
+
+* Bump version to 1.0.0-beta1 to indicate graduation from being experimental. See [1846](https://github.com/WordPress/performance/pull/1846).
+* Use CSS range syntax in media queries. ([1833](https://github.com/WordPress/performance/pull/1833))
+
 = 0.4.1 =
 
 **Bug Fixes**

Index: class-image-prioritizer-img-tag-visitor.php
===================================================================
--- class-image-prioritizer-img-tag-visitor.php	(revision 3238281)
+++ class-image-prioritizer-img-tag-visitor.php	(working copy)
@@ -146,21 +146,44 @@
 			$processor->remove_attribute( 'fetchpriority' );
 		}
 
-		// Ensure that sizes=auto is set properly.
-		$sizes = $processor->get_attribute( 'sizes' );
-		if ( is_string( $sizes ) ) {
+		// Ensure that sizes is set properly when it is a responsive image (it has a srcset attribute).
+		if ( is_string( $processor->get_attribute( 'srcset' ) ) ) {
+			$sizes = $processor->get_attribute( 'sizes' );
+			if ( ! is_string( $sizes ) ) {
+				$sizes = '';
+			}
+
 			$is_lazy  = 'lazy' === $this->get_attribute_value( $processor, 'loading' );
 			$has_auto = $this->sizes_attribute_includes_valid_auto( $sizes );
 
 			if ( $is_lazy && ! $has_auto ) {
-				$processor->set_attribute( 'sizes', "auto, $sizes" );
+				$new_sizes = 'auto';
+				if ( '' !== trim( $sizes, " \t\f\r\n" ) ) {
+					$new_sizes .= ', ';
+				}
+				$sizes = $new_sizes . $sizes;
 			} elseif ( ! $is_lazy && $has_auto ) {
 				// Remove auto from the beginning of the list.
-				$processor->set_attribute(
-					'sizes',
-					(string) preg_replace( '/^[ \t\f\r\n]*auto[ \t\f\r\n]*(,[ \t\f\r\n]*)?/i', '', $sizes )
-				);
+				$sizes = (string) preg_replace( '/^[ \t\f\r\n]*auto[ \t\f\r\n]*(,[ \t\f\r\n]*)?/i', '', $sizes );
 			}
+
+			// Compute more accurate sizes when it isn't lazy-loaded and sizes=auto isn't taking care of it.
+			if ( ! $is_lazy ) {
+				$computed_sizes = $this->compute_sizes( $context );
+				if ( count( $computed_sizes ) > 0 ) {
+					$new_sizes = join( ', ', $computed_sizes );
+
+					// Preserve the original sizes as a fallback when URL Metrics are missing from one or more viewport group.
+					// Note that when all groups are populated, the media features will span all possible viewport widths from
+					// zero to infinity, so there is no need to include the original sizes since they will never match.
+					if ( '' !== $sizes && ! $context->url_metric_group_collection->is_every_group_populated() ) {
+						$new_sizes .= ", $sizes";
+					}
+					$sizes = $new_sizes;
+				}
+			}
+
+			$processor->set_attribute( 'sizes', $sizes );
 		}
 
 		$parent_tag = $this->get_parent_tag_name( $context );
@@ -385,4 +408,38 @@
 			return 'auto' === $sizes_attr || str_starts_with( $sizes_attr, 'auto,' );
 		}
 	}
+
+	/**
+	 * Computes responsive sizes for the current element based on its boundingClientRect width captured in URL Metrics.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @param OD_Tag_Visitor_Context $context Context.
+	 * @return non-empty-string[] Computed sizes.
+	 */
+	private function compute_sizes( OD_Tag_Visitor_Context $context ): array {
+		$sizes = array();
+
+		$xpath = $context->processor->get_xpath();
+		foreach ( $context->url_metric_group_collection as $group ) {
+			// Obtain the maximum width that the image appears among all URL Metrics collected for this viewport group.
+			$element_max_width = 0;
+			foreach ( $group->get_xpath_elements_map()[ $xpath ] ?? array() as $element ) {
+				$element_max_width = max( $element_max_width, $element->get_bounding_client_rect()['width'] );
+			}
+
+			// Use the maximum width as the size for image in this breakpoint.
+			if ( $element_max_width > 0 ) {
+				$size          = sprintf( '%dpx', $element_max_width );
+				$media_feature = od_generate_media_query( $group->get_minimum_viewport_width(), $group->get_maximum_viewport_width() );
+				if ( null !== $media_feature ) {
+					// Note: The null case only happens when a site has filtered od_breakpoint_max_widths to be an empty array, meaning there is only one viewport group.
+					$size = "$media_feature $size";
+				}
+				$sizes[] = $size;
+			}
+		}
+
+		return $sizes;
+	}
 }
Index: load.php
===================================================================
--- load.php	(revision 3238281)
+++ load.php	(working copy)
@@ -6,7 +6,7 @@
  * Requires at least: 6.6
  * Requires PHP: 7.2
  * Requires Plugins: optimization-detective
- * Version: 0.3.1
+ * Version: 1.0.0-beta1
  * Author: WordPress Performance Team
  * Author URI: https://make.wordpress.org/performance/
  * License: GPLv2 or later
@@ -72,7 +72,7 @@
 	}
 )(
 	'image_prioritizer_pending_plugin',
-	'0.3.1',
+	'1.0.0-beta1',
 	static function ( string $version ): void {
 		if ( defined( 'IMAGE_PRIORITIZER_VERSION' ) ) {
 			return;
Index: readme.txt
===================================================================
--- readme.txt	(revision 3238281)
+++ readme.txt	(working copy)
@@ -2,12 +2,12 @@
 
 Contributors: wordpressdotorg
 Tested up to: 6.7
-Stable tag:   0.3.1
+Stable tag:   1.0.0-beta1
 License:      GPLv2 or later
 License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 Tags:         performance, optimization, image, lcp, lazy-load
 
-Prioritizes the loading of images and videos based on how visible they are to actual visitors; adds fetchpriority and applies lazy loading.
+Prioritizes the loading of images and videos based on how they appear to actual visitors: adds fetchpriority, preloads, lazy-loads, and sets sizes.
 
 == Description ==
 
@@ -15,7 +15,7 @@
 
 The current optimizations include:
 
-1. Add breakpoint-specific `fetchpriority=high` preload links (`LINK[rel=preload]`) for image URLs of LCP elements:
+1. Add breakpoint-specific `fetchpriority=high` preload links (both as `LINK[rel=preload]` elements and `Link` response headers) for image URLs of LCP elements:
    1. An `IMG` element, including the `srcset`/`sizes` attributes supplied as `imagesrcset`/`imagesizes` on the `LINK`.
    2. The first `SOURCE` element with a `type` attribute in a `PICTURE` element. (Art-directed `PICTURE` elements using media queries are not supported.)
    3. An element with a CSS `background-image` inline `style` attribute.
@@ -27,7 +27,9 @@
    1. Apply lazy loading to `IMG` tags based on whether they appear in any breakpoint’s initial viewport.
    2. Implement lazy loading of CSS background images added via inline `style` attributes.
    3. Lazy-load `VIDEO` tags by setting the appropriate attributes based on whether they appear in the initial viewport. If a `VIDEO` is the LCP element, it gets `preload=auto`; if it is in an initial viewport, the `preload=metadata` default is left; if it is not in an initial viewport, it gets `preload=none`. Lazy-loaded videos also get initial `preload`, `autoplay`, and `poster` attributes restored when the `VIDEO` is going to enter the viewport.
-5. Ensure that [`sizes=auto`](https://make.wordpress.org/core/2024/10/18/auto-sizes-for-lazy-loaded-images-in-wordpress-6-7/) is added to all lazy-loaded `IMG` elements.
+5. Responsive image sizes:
+   1. Compute the `sizes` attribute using the widths of an image collected from URL Metrics for each breakpoint (when not lazy-loaded since then handled by `sizes=auto`).
+   2. Ensure [`sizes=auto`](https://make.wordpress.org/core/2024/10/18/auto-sizes-for-lazy-loaded-images-in-wordpress-6-7/) is set on `IMG` tags after setting correct lazy-loading (above).
 6. Reduce the size of the `poster` image of a `VIDEO` from full size to the size appropriate for the maximum width of the video (on desktop).
 
 **This plugin requires the [Optimization Detective](https://wordpress.org/plugins/optimization-detective/) plugin as a dependency.** Please refer to that plugin for additional background on how this plugin works as well as additional developer options.
@@ -70,6 +72,13 @@
 
 == Changelog ==
 
+= 1.0.0-beta1 =
+
+**Enhancements**
+
+* Bump version to 1.0.0-beta1 to indicate graduation from being experimental. See [1846](https://github.com/WordPress/performance/pull/1846).
+* Compute responsive `sizes` attribute based on the `width` from the `boundingClientRect` in captured URL Metrics. ([1840](https://github.com/WordPress/performance/pull/1840))
+
 = 0.3.1 =
 
 **Bug Fixes**

Index: class-od-data-validation-exception.php
===================================================================
--- class-od-data-validation-exception.php	(revision 3238281)
+++ class-od-data-validation-exception.php	(working copy)
@@ -16,6 +16,5 @@
  * Exception thrown when failing to validate URL Metrics data.
  *
  * @since 0.1.0
- * @access private
  */
 final class OD_Data_Validation_Exception extends Exception {}
Index: class-od-element.php
===================================================================
--- class-od-element.php	(revision 3238281)
+++ class-od-element.php	(working copy)
@@ -21,7 +21,6 @@
  * @todo The above implements tag should account for additional undefined keys which can be supplied by extending the element schema. May depend on <https://github.com/phpstan/phpstan/issues/8438>.
  *
  * @since 0.7.0
- * @access private
  */
 class OD_Element implements ArrayAccess, JsonSerializable {
 
Index: class-od-html-tag-processor.php
===================================================================
--- class-od-html-tag-processor.php	(revision 3238281)
+++ class-od-html-tag-processor.php	(working copy)
@@ -16,7 +16,6 @@
  * Extension to WP_HTML_Tag_Processor that supports injecting HTML and obtaining XPath for the current tag.
  *
  * @since 0.1.1
- * @access private
  */
 final class OD_HTML_Tag_Processor extends WP_HTML_Tag_Processor {
 
@@ -149,7 +148,7 @@
 	 * Open stack tags.
 	 *
 	 * @since 0.4.0
-	 * @var string[]
+	 * @var non-empty-string[]
 	 */
 	private $open_stack_tags = array();
 
@@ -160,7 +159,7 @@
 	 * are children of the `BODY` tag. This is used in {@see self::get_xpath()}.
 	 *
 	 * @since 1.0.0
-	 * @var array<array<string, string>>
+	 * @var array<array<non-empty-string, string>>
 	 */
 	private $open_stack_attributes = array();
 
@@ -168,7 +167,7 @@
 	 * Open stack indices.
 	 *
 	 * @since 0.4.0
-	 * @var int[]
+	 * @var non-negative-int[]
 	 */
 	private $open_stack_indices = array();
 
@@ -181,7 +180,7 @@
 	 * populated back into `$this->open_stack_tags` and `$this->open_stack_indices`.
 	 *
 	 * @since 0.4.0
-	 * @var array<string, array{tags: string[], attributes: array<array<string, string>>, indices: int[]}>
+	 * @var array<string, array{tags: non-empty-string[], attributes: array<array<non-empty-string, string>>, indices: non-negative-int[]}>
 	 */
 	private $bookmarked_open_stacks = array();
 
@@ -221,7 +220,7 @@
 	 * Mapping of bookmark name to a list of HTML strings which will be inserted at the time get_updated_html() is called.
 	 *
 	 * @since 0.4.0
-	 * @var array<string, string[]>
+	 * @var array<non-empty-string, string[]>
 	 */
 	private $buffered_text_replacements = array();
 
@@ -238,7 +237,7 @@
 	 * Count for the number of times that the cursor was moved.
 	 *
 	 * @since 0.6.0
-	 * @var int
+	 * @var non-negative-int
 	 * @see self::next_token()
 	 * @see self::seek()
 	 */
@@ -292,7 +291,7 @@
 	 * @see WP_HTML_Processor::expects_closer()
 	 * @since 0.4.0
 	 *
-	 * @param string|null $tag_name Tag name, if not provided then the current tag is used. Optional.
+	 * @param non-empty-string|null $tag_name Tag name, if not provided then the current tag is used. Optional.
 	 * @return bool Whether to expect a closer for the tag.
 	 */
 	public function expects_closer( ?string $tag_name = null ): bool {
@@ -336,6 +335,11 @@
 		if ( null === $tag_name || $this->get_token_type() !== '#tag' ) {
 			return true;
 		}
+		/**
+		 * Tag name.
+		 *
+		 * @var non-empty-string $tag_name
+		 */
 
 		if ( $this->previous_tag_without_closer ) {
 			array_pop( $this->open_stack_tags );
@@ -422,7 +426,7 @@
 	 * @see self::next_token()
 	 * @see self::seek()
 	 *
-	 * @return int Count of times the cursor has moved.
+	 * @return non-negative-int Count of times the cursor has moved.
 	 */
 	public function get_cursor_move_count(): int {
 		return $this->cursor_move_count;
@@ -458,8 +462,8 @@
 	 *
 	 * @since 0.4.0
 	 *
-	 * @param string      $name  Meta attribute name.
-	 * @param string|true $value Value.
+	 * @param non-empty-string $name  Meta attribute name.
+	 * @param string|true      $value Value.
 	 * @return bool Whether an attribute was set.
 	 */
 	public function set_meta_attribute( string $name, $value ): bool {
@@ -489,7 +493,7 @@
 	 * @since 0.4.0
 	 * @see WP_HTML_Processor::get_current_depth()
 	 *
-	 * @return int Nesting-depth of current location in the document.
+	 * @return non-negative-int Nesting-depth of current location in the document.
 	 */
 	public function get_current_depth(): int {
 		return count( $this->open_stack_tags );
@@ -565,7 +569,7 @@
 	 * @since 0.4.0
 	 * @since 0.9.0 Renamed from get_breadcrumbs() to get_indexed_breadcrumbs().
 	 *
-	 * @return Generator<array{string, int, array<string, string>}> Breadcrumb.
+	 * @return Generator<array{non-empty-string, non-negative-int, array<non-empty-string, string>}> Breadcrumb.
 	 */
 	private function get_indexed_breadcrumbs(): Generator {
 		foreach ( $this->open_stack_tags as $i => $breadcrumb_tag_name ) {
@@ -584,7 +588,7 @@
 	 *
 	 * @since 1.0.0
 	 *
-	 * @return array<string, string> Disambiguating attributes.
+	 * @return array<non-empty-string, string> Disambiguating attributes.
 	 */
 	private function get_disambiguating_attributes(): array {
 		$attributes = array();
@@ -617,7 +621,7 @@
 	 * @since 0.9.0
 	 * @see WP_HTML_Processor::get_breadcrumbs()
 	 *
-	 * @return string[] Array of tag names representing path to matched node.
+	 * @return non-empty-string[] Array of tag names representing path to matched node.
 	 */
 	public function get_breadcrumbs(): array {
 		return $this->open_stack_tags;
Index: class-od-link-collection.php
===================================================================
--- class-od-link-collection.php	(revision 3238281)
+++ class-od-link-collection.php	(working copy)
@@ -18,7 +18,7 @@
  * @phpstan-type Link array{
  *                   attributes: LinkAttributes,
  *                   minimum_viewport_width: int<0, max>|null,
- *                   maximum_viewport_width: positive-int|null
+ *                   maximum_viewport_width: int<1, max>|null
  *               }
  *
  * @phpstan-type LinkAttributes array{
@@ -37,7 +37,6 @@
  *
  * @since 0.3.0
  * @since 0.4.0 Renamed from OD_Preload_Link_Collection.
- * @access private
  */
 final class OD_Link_Collection implements Countable {
 
@@ -44,6 +43,8 @@
 	/**
 	 * Links grouped by rel type.
 	 *
+	 * @since 0.4.0
+	 *
 	 * @var array<string, Link[]>
 	 */
 	private $links_by_rel = array();
@@ -51,11 +52,13 @@
 	/**
 	 * Adds link.
 	 *
+	 * @since 0.3.0
+	 *
 	 * @phpstan-param LinkAttributes $attributes
 	 *
-	 * @param array             $attributes             Attributes.
-	 * @param int<0, max>|null  $minimum_viewport_width Minimum width or null if not bounded or relevant.
-	 * @param positive-int|null $maximum_viewport_width Maximum width or null if not bounded (i.e. infinity) or relevant.
+	 * @param array            $attributes             Attributes.
+	 * @param int<0, max>|null $minimum_viewport_width Minimum width (exclusive) or null if not bounded or relevant.
+	 * @param int<1, max>|null $maximum_viewport_width Maximum width (inclusive) or null if not bounded (i.e. infinity) or relevant.
 	 *
 	 * @throws InvalidArgumentException When invalid arguments are provided.
 	 */
@@ -111,6 +114,8 @@
 	 * When two links are identical except for their minimum/maximum widths which are also consecutive, then merge them
 	 * together. Also, add media attributes to the links.
 	 *
+	 * @since 0.4.0
+	 *
 	 * @return LinkAttributes[] Prepared links with adjacent-duplicates merged together and media attributes added.
 	 */
 	private function get_prepared_links(): array {
@@ -133,6 +138,8 @@
 	/**
 	 * Merges consecutive links.
 	 *
+	 * @since 0.4.0
+	 *
 	 * @param Link[] $links Links.
 	 * @return LinkAttributes[] Merged consecutive links.
 	 */
@@ -194,9 +201,9 @@
 					&&
 					is_int( $last_link['maximum_viewport_width'] )
 					&&
-					$last_link['maximum_viewport_width'] + 1 === $link['minimum_viewport_width']
+					$last_link['maximum_viewport_width'] === $link['minimum_viewport_width']
 				) {
-					$last_link['maximum_viewport_width'] = max( $last_link['maximum_viewport_width'], $link['maximum_viewport_width'] );
+					$last_link['maximum_viewport_width'] = null === $link['maximum_viewport_width'] ? null : max( $last_link['maximum_viewport_width'], $link['maximum_viewport_width'] );
 
 					// Update the last link with the new maximum viewport width.
 					$carry[ count( $carry ) - 1 ] = $last_link;
@@ -228,6 +235,8 @@
 	/**
 	 * Gets the HTML for the link tags.
 	 *
+	 * @since 0.3.0
+	 *
 	 * @return string Link tags HTML.
 	 */
 	public function get_html(): string {
@@ -249,15 +258,32 @@
 	/**
 	 * Constructs the Link HTTP response header.
 	 *
-	 * @return string|null Link HTTP response header, or null if there are none.
+	 * @since 0.4.0
+	 *
+	 * @return non-empty-string|null Link HTTP response header, or null if there are none.
 	 */
 	public function get_response_header(): ?string {
 		$link_headers = array();
 
 		foreach ( $this->get_prepared_links() as $link ) {
-			// The about:blank is present since a Link without a reference-uri is invalid so any imagesrcset would otherwise not get downloaded.
-			$link['href'] = isset( $link['href'] ) ? esc_url_raw( $link['href'] ) : 'about:blank';
-			$link_header  = '<' . $link['href'] . '>';
+			if ( isset( $link['href'] ) ) {
+				$decoded_url = urldecode( $link['href'] );
+
+				// Encode characters not allowed in a URL per RFC 3986 (anything that is not among the reserved and unreserved characters).
+				$encoded_url  = preg_replace_callback(
+					'/[^A-Za-z0-9\-._~:\/?#\[\]@!$&\'()*+,;=]/',
+					static function ( $matches ) {
+						return rawurlencode( $matches[0] );
+					},
+					$decoded_url
+				);
+				$link['href'] = esc_url_raw( $encoded_url ?? '' );
+			} else {
+				// The about:blank is present since a Link without a reference-uri is invalid so any imagesrcset would otherwise not get downloaded.
+				$link['href'] = 'about:blank';
+			}
+
+			$link_header = '<' . $link['href'] . '>';
 			unset( $link['href'] );
 			foreach ( $link as $name => $value ) {
 				/*
@@ -287,6 +313,8 @@
 	/**
 	 * Counts the links.
 	 *
+	 * @since 0.3.0
+	 *
 	 * @return non-negative-int Link count.
 	 */
 	public function count(): int {
Index: class-od-tag-visitor-context.php
===================================================================
--- class-od-tag-visitor-context.php	(revision 3238281)
+++ class-od-tag-visitor-context.php	(working copy)
@@ -17,7 +17,11 @@
  *
  * @since 0.4.0
  *
- * @property-read OD_URL_Metric_Group_Collection $url_metrics_group_collection Deprecated property accessed via magic getter. Use the url_metric_group_collection property instead.
+ * @property-read OD_HTML_Tag_Processor          $processor                    HTML tag processor.
+ * @property-read OD_URL_Metric_Group_Collection $url_metric_group_collection  URL Metric group collection.
+ * @property-read OD_Link_Collection             $link_collection              Link collection.
+ * @property-read positive-int|null              $url_metrics_id               ID for the od_url_metrics post which provided the URL Metrics in the collection.
+ * @property-read OD_URL_Metric_Group_Collection $url_metrics_group_collection Deprecated alias for the $url_metric_group_collection property.
  */
 final class OD_Tag_Visitor_Context {
 
@@ -26,9 +30,8 @@
 	 *
 	 * @since 0.4.0
 	 * @var OD_HTML_Tag_Processor
-	 * @readonly
 	 */
-	public $processor;
+	private $processor;
 
 	/**
 	 * URL Metric group collection.
@@ -35,9 +38,8 @@
 	 *
 	 * @since 0.4.0
 	 * @var OD_URL_Metric_Group_Collection
-	 * @readonly
 	 */
-	public $url_metric_group_collection;
+	private $url_metric_group_collection;
 
 	/**
 	 * Link collection.
@@ -44,13 +46,24 @@
 	 *
 	 * @since 0.4.0
 	 * @var OD_Link_Collection
-	 * @readonly
 	 */
-	public $link_collection;
+	private $link_collection;
 
 	/**
+	 * ID for the od_url_metrics post which provided the URL Metrics in the collection.
+	 *
+	 * May be null if no post has been created yet.
+	 *
+	 * @since 1.0.0
+	 * @var positive-int|null
+	 */
+	private $url_metrics_id;
+
+	/**
 	 * Visited tag state.
 	 *
+	 * Important: This object is not exposed directly by the getter. It is only exposed via {@see self::track_tag()}.
+	 *
 	 * @since 1.0.0
 	 * @var OD_Visited_Tag_State
 	 */
@@ -65,12 +78,14 @@
 	 * @param OD_URL_Metric_Group_Collection $url_metric_group_collection URL Metric group collection.
 	 * @param OD_Link_Collection             $link_collection             Link collection.
 	 * @param OD_Visited_Tag_State           $visited_tag_state           Visited tag state.
+	 * @param positive-int|null              $url_metrics_id              ID for the od_url_metrics post which provided the URL Metrics in the collection. May be null if no post has been created yet.
 	 */
-	public function __construct( OD_HTML_Tag_Processor $processor, OD_URL_Metric_Group_Collection $url_metric_group_collection, OD_Link_Collection $link_collection, OD_Visited_Tag_State $visited_tag_state ) {
+	public function __construct( OD_HTML_Tag_Processor $processor, OD_URL_Metric_Group_Collection $url_metric_group_collection, OD_Link_Collection $link_collection, OD_Visited_Tag_State $visited_tag_state, ?int $url_metrics_id ) {
 		$this->processor                   = $processor;
 		$this->url_metric_group_collection = $url_metric_group_collection;
 		$this->link_collection             = $link_collection;
 		$this->visited_tag_state           = $visited_tag_state;
+		$this->url_metrics_id              = $url_metrics_id;
 	}
 
 	/**
@@ -85,39 +100,50 @@
 	}
 
 	/**
-	 * Gets deprecated property.
+	 * Gets a property.
 	 *
 	 * @since 0.7.0
-	 * @todo Remove this when no plugins are possibly referring to the url_metrics_group_collection property anymore.
 	 *
 	 * @param string $name Property name.
-	 * @return OD_URL_Metric_Group_Collection URL Metric group collection.
+	 * @return mixed Property value.
 	 *
 	 * @throws Error When property is unknown.
 	 */
-	public function __get( string $name ): OD_URL_Metric_Group_Collection {
-		if ( 'url_metrics_group_collection' === $name ) {
-			_doing_it_wrong(
-				__CLASS__ . '::$url_metrics_group_collection',
-				esc_html(
-					sprintf(
-						/* translators: %s is class member variable name */
-						__( 'Use %s instead.', 'optimization-detective' ),
-						__CLASS__ . '::$url_metric_group_collection'
+	public function __get( string $name ) {
+		// Note that there is intentionally not a case for 'visited_tag_state'.
+		switch ( $name ) {
+			case 'processor':
+				return $this->processor;
+			case 'link_collection':
+				return $this->link_collection;
+			case 'url_metrics_id':
+				return $this->url_metrics_id;
+			case 'url_metric_group_collection':
+				return $this->url_metric_group_collection;
+			case 'url_metrics_group_collection':
+				// TODO: Remove this when no plugins are possibly referring to the url_metrics_group_collection property anymore.
+				_doing_it_wrong(
+					esc_html( __CLASS__ . '::$' . $name ),
+					esc_html(
+						sprintf(
+							/* translators: %s is class member variable name */
+							__( 'Use %s instead.', 'optimization-detective' ),
+							__CLASS__ . '::$url_metric_group_collection'
+						)
+					),
+					'optimization-detective 0.7.0'
+				);
+				return $this->url_metric_group_collection;
+			default:
+				throw new Error(
+					esc_html(
+						sprintf(
+							/* translators: %s is class member variable name */
+							__( 'Unknown property %s.', 'optimization-detective' ),
+							__CLASS__ . '::$' . $name
+						)
 					)
-				),
-				'optimization-detective 0.7.0'
-			);
-			return $this->url_metric_group_collection;
+				);
 		}
-		throw new Error(
-			esc_html(
-				sprintf(
-					/* translators: %s is class member variable name */
-					__( 'Unknown property %s.', 'optimization-detective' ),
-					__CLASS__ . '::$' . $name
-				)
-			)
-		);
 	}
 }
Index: class-od-tag-visitor-registry.php
===================================================================
--- class-od-tag-visitor-registry.php	(revision 3238281)
+++ class-od-tag-visitor-registry.php	(working copy)
@@ -20,7 +20,6 @@
  * @implements IteratorAggregate<string, TagVisitorCallback>
  *
  * @since 0.3.0
- * @access private
  */
 final class OD_Tag_Visitor_Registry implements Countable, IteratorAggregate {
 
@@ -27,7 +26,9 @@
 	/**
 	 * Visitors.
 	 *
-	 * @var array<string, TagVisitorCallback>
+	 * @since 0.3.0
+	 *
+	 * @var array<non-empty-string, TagVisitorCallback>
 	 */
 	private $visitors = array();
 
@@ -34,10 +35,12 @@
 	/**
 	 * Registers a tag visitor.
 	 *
+	 * @since 0.3.0
+	 *
 	 * @phpstan-param TagVisitorCallback $tag_visitor_callback
 	 *
-	 * @param string   $id                   Identifier for the tag visitor.
-	 * @param callable $tag_visitor_callback Tag visitor callback.
+	 * @param non-empty-string $id                   Identifier for the tag visitor.
+	 * @param callable         $tag_visitor_callback Tag visitor callback.
 	 */
 	public function register( string $id, callable $tag_visitor_callback ): void {
 		$this->visitors[ $id ] = $tag_visitor_callback;
@@ -46,7 +49,9 @@
 	/**
 	 * Determines if a visitor has been registered.
 	 *
-	 * @param string $id Identifier for the tag visitor.
+	 * @since 0.3.0
+	 *
+	 * @param non-empty-string $id Identifier for the tag visitor.
 	 * @return bool Whether registered.
 	 */
 	public function is_registered( string $id ): bool {
@@ -56,7 +61,9 @@
 	/**
 	 * Gets a registered visitor.
 	 *
-	 * @param string $id Identifier for the tag visitor.
+	 * @since 0.3.0
+	 *
+	 * @param non-empty-string $id Identifier for the tag visitor.
 	 * @return TagVisitorCallback|null Whether registered.
 	 */
 	public function get_registered( string $id ): ?callable {
@@ -69,7 +76,9 @@
 	/**
 	 * Unregisters a tag visitor.
 	 *
-	 * @param string $id Identifier for the tag visitor.
+	 * @since 0.3.0
+	 *
+	 * @param non-empty-string $id Identifier for the tag visitor.
 	 * @return bool Whether a tag visitor was unregistered.
 	 */
 	public function unregister( string $id ): bool {
@@ -83,6 +92,8 @@
 	/**
 	 * Returns an iterator for the URL Metrics in the group.
 	 *
+	 * @since 0.3.0
+	 *
 	 * @return ArrayIterator<string, TagVisitorCallback> ArrayIterator for tag visitors.
 	 */
 	public function getIterator(): ArrayIterator {
@@ -92,6 +103,8 @@
 	/**
 	 * Counts the URL Metrics in the group.
 	 *
+	 * @since 0.3.0
+	 *
 	 * @return int<0, max> URL Metric count.
 	 */
 	public function count(): int {
Index: class-od-url-metric-group-collection.php
===================================================================
--- class-od-url-metric-group-collection.php	(revision 3238281)
+++ class-od-url-metric-group-collection.php	(working copy)
@@ -18,7 +18,6 @@
  * @implements IteratorAggregate<int, OD_URL_Metric_Group>
  *
  * @since 0.1.0
- * @access private
  */
 final class OD_URL_Metric_Group_Collection implements Countable, IteratorAggregate, JsonSerializable {
 
@@ -31,6 +30,7 @@
 	 * even to when there are zero breakpoints: there will still be one group
 	 * in this case, in which every single URL Metric is added.
 	 *
+	 * @since 0.1.0
 	 * @var OD_URL_Metric_Group[]
 	 * @phpstan-var non-empty-array<OD_URL_Metric_Group>
 	 */
@@ -47,18 +47,15 @@
 	/**
 	 * Breakpoints in max widths.
 	 *
-	 * Valid values are from 1 to PHP_INT_MAX - 1. This is because:
+	 * A breakpoint must be greater than zero because a viewport group's maximum viewport width has a minimum (inclusive)
+	 * value of 1, and the breakpoints are used as the maximum viewport widths for the viewport groups, with the addition of
+	 * a final viewport group which has a maximum viewport width of infinity.
 	 *
-	 * 1. It doesn't make sense for there to be a viewport width of zero, so the first breakpoint (max width) must be at least 1.
-	 * 2. After the last breakpoint, the final breakpoint group is set to be spanning one plus the last breakpoint max width up
-	 *    until PHP_INT_MAX. So a breakpoint cannot be PHP_INT_MAX because then the minimum viewport width for the final group
-	 *    would end up being larger than PHP_INT_MAX.
-	 *
 	 * This array may be empty in which case there are no responsive breakpoints and all URL Metrics are collected in a
 	 * single group.
 	 *
-	 * @var int[]
-	 * @phpstan-var positive-int[]
+	 * @since 0.1.0
+	 * @var positive-int[]
 	 */
 	private $breakpoints;
 
@@ -65,8 +62,8 @@
 	/**
 	 * Sample size for URL Metrics for a given breakpoint.
 	 *
-	 * @var int
-	 * @phpstan-var positive-int
+	 * @since 0.1.0
+	 * @var int<1, max>
 	 */
 	private $sample_size;
 
@@ -75,8 +72,8 @@
 	 *
 	 * A freshness age of zero means a URL Metric will always be considered stale.
 	 *
-	 * @var int
-	 * @phpstan-var 0|positive-int
+	 * @since 0.1.0
+	 * @var int<0, max>
 	 */
 	private $freshness_ttl;
 
@@ -83,6 +80,7 @@
 	/**
 	 * Result cache.
 	 *
+	 * @since 0.3.0
 	 * @var array{
 	 *          get_group_for_viewport_width?: array<int, OD_URL_Metric_Group>,
 	 *          is_every_group_populated?: bool,
@@ -91,7 +89,7 @@
 	 *          get_groups_by_lcp_element?: array<string, OD_URL_Metric_Group[]>,
 	 *          get_common_lcp_element?: OD_Element|null,
 	 *          get_all_element_max_intersection_ratios?: array<string, float>,
-	 *          get_xpath_elements_map?: array<string, non-empty-array<int, OD_Element>>,
+	 *          get_xpath_elements_map?: array<string, non-empty-array<non-negative-int, OD_Element>>,
 	 *          get_all_elements_positioned_in_any_initial_viewport?: array<string, bool>,
 	 *      }
 	 */
@@ -100,8 +98,14 @@
 	/**
 	 * Constructor.
 	 *
+	 * @since 0.1.0
+	 *
 	 * @throws InvalidArgumentException When an invalid argument is supplied.
 	 *
+	 * @phpstan-param positive-int[] $breakpoints
+	 * @phpstan-param int<1, max>    $sample_size
+	 * @phpstan-param int<0, max>    $freshness_ttl
+	 *
 	 * @param OD_URL_Metric[]  $url_metrics   URL Metrics.
 	 * @param non-empty-string $current_etag  The current ETag.
 	 * @param int[]            $breakpoints   Breakpoints in max widths.
@@ -127,13 +131,13 @@
 		sort( $breakpoints );
 		$breakpoints = array_values( array_unique( $breakpoints, SORT_NUMERIC ) );
 		foreach ( $breakpoints as $breakpoint ) {
-			if ( ! is_int( $breakpoint ) || $breakpoint < 1 || PHP_INT_MAX === $breakpoint ) {
+			if ( ! is_int( $breakpoint ) || $breakpoint < 1 ) {
 				throw new InvalidArgumentException(
 					esc_html(
 						sprintf(
 							/* translators: %d is the invalid breakpoint */
 							__(
-								'Each of the breakpoints must be greater than zero and less than PHP_INT_MAX, but encountered: %d',
+								'Each of the breakpoints must be greater than zero, but encountered: %d',
 								'optimization-detective'
 							),
 							$breakpoint
@@ -196,6 +200,39 @@
 	}
 
 	/**
+	 * Gets the breakpoints in max widths.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @return positive-int[] Breakpoints in max widths.
+	 */
+	public function get_breakpoints(): array {
+		return $this->breakpoints;
+	}
+
+	/**
+	 * Gets the sample size for URL Metrics for a given breakpoint.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @return int<1, max> Sample size for URL Metrics for a given breakpoint.
+	 */
+	public function get_sample_size(): int {
+		return $this->sample_size;
+	}
+
+	/**
+	 * Gets the freshness age (TTL) for a given URL Metric..
+	 *
+	 * @since 1.0.0
+	 *
+	 * @return int<0, max> Freshness age (TTL) for a given URL Metric.
+	 */
+	public function get_freshness_ttl(): int {
+		return $this->freshness_ttl;
+	}
+
+	/**
 	 * Gets the first URL Metric group.
 	 *
 	 * This group normally represents viewports for mobile devices. This group always has a minimum viewport width of 0
@@ -215,7 +252,7 @@
 	 *
 	 * This group normally represents viewports for desktop devices.  This group always has a minimum viewport width
 	 * defined as one greater than the largest breakpoint returned by {@see od_get_breakpoint_max_widths()}.
-	 * The maximum viewport is always `PHP_INT_MAX`, or in other words it is unbounded.
+	 * The maximum viewport width of this group is always `null`, or in other words it is unbounded.
 	 *
 	 * @since 0.7.0
 	 *
@@ -244,13 +281,13 @@
 	 * @return OD_URL_Metric_Group[] Groups.
 	 */
 	private function create_groups(): array {
-		$groups    = array();
-		$min_width = 0;
-		foreach ( $this->breakpoints as $max_width ) {
-			$groups[]  = new OD_URL_Metric_Group( array(), $min_width, $max_width, $this->sample_size, $this->freshness_ttl, $this );
-			$min_width = $max_width + 1;
+		$groups              = array();
+		$min_width_exclusive = 0;
+		foreach ( $this->breakpoints as $max_width_inclusive ) {
+			$groups[]            = new OD_URL_Metric_Group( array(), $min_width_exclusive, $max_width_inclusive, $this->sample_size, $this->freshness_ttl, $this );
+			$min_width_exclusive = $max_width_inclusive;
 		}
-		$groups[] = new OD_URL_Metric_Group( array(), $min_width, PHP_INT_MAX, $this->sample_size, $this->freshness_ttl, $this );
+		$groups[] = new OD_URL_Metric_Group( array(), $min_width_exclusive, null, $this->sample_size, $this->freshness_ttl, $this );
 		return $groups;
 	}
 
@@ -272,7 +309,7 @@
 			}
 		}
 		// @codeCoverageIgnoreStart
-		// In practice this exception should never get thrown because create_groups() creates groups from a minimum of 0 to a maximum of PHP_INT_MAX.
+		// In practice this exception should never get thrown because create_groups() creates groups from a minimum of 0 to an unbounded maximum.
 		throw new InvalidArgumentException(
 			esc_html__( 'No group available to add URL Metric to.', 'optimization-detective' )
 		);
@@ -285,7 +322,7 @@
 	 * @since 0.1.0
 	 * @throws InvalidArgumentException When there is no group for the provided viewport width. This would only happen if a negative width is provided.
 	 *
-	 * @param int $viewport_width Viewport width.
+	 * @param positive-int $viewport_width Viewport width.
 	 * @return OD_URL_Metric_Group URL Metric group for the viewport width.
 	 */
 	public function get_group_for_viewport_width( int $viewport_width ): OD_URL_Metric_Group {
@@ -300,7 +337,7 @@
 				}
 			}
 			// @codeCoverageIgnoreStart
-			// In practice this exception should never get thrown because create_groups() creates groups from a minimum of 0 to a maximum of PHP_INT_MAX.
+			// In practice this exception should never get thrown because create_groups() creates groups from a minimum of 0 to an unbounded maximum.
 			throw new InvalidArgumentException(
 				esc_html(
 					sprintf(
@@ -495,7 +532,7 @@
 	 *
 	 * @since 0.7.0
 	 *
-	 * @return array<string, non-empty-array<int, OD_Element>> Keys are XPaths and values are the element instances.
+	 * @return array<string, non-empty-array<non-negative-int, OD_Element>> Keys are XPaths and values are the element instances.
 	 */
 	public function get_xpath_elements_map(): array {
 		if ( array_key_exists( __FUNCTION__, $this->result_cache ) ) {
@@ -667,8 +704,8 @@
 	 *             every_group_populated: bool,
 	 *             groups: array<int, array{
 	 *                 lcp_element: ?OD_Element,
-	 *                 minimum_viewport_width: 0|positive-int,
-	 *                 maximum_viewport_width: positive-int,
+	 *                 minimum_viewport_width: int<0, max>,
+	 *                 maximum_viewport_width: int<1, max>|null,
 	 *                 complete: bool,
 	 *                 url_metrics: OD_URL_Metric[]
 	 *             }>
Index: class-od-url-metric-group.php
===================================================================
--- class-od-url-metric-group.php	(revision 3238281)
+++ class-od-url-metric-group.php	(working copy)
@@ -18,7 +18,6 @@
  * @implements IteratorAggregate<int, OD_URL_Metric>
  *
  * @since 0.1.0
- * @access private
  */
 final class OD_URL_Metric_Group implements IteratorAggregate, Countable, JsonSerializable {
 
@@ -32,22 +31,20 @@
 	private $url_metrics;
 
 	/**
-	 * Minimum possible viewport width for the group (inclusive).
+	 * Minimum possible viewport width for the group (exclusive).
 	 *
 	 * @since 0.1.0
 	 *
-	 * @var int
-	 * @phpstan-var 0|positive-int
+	 * @var int<0, max>
 	 */
 	private $minimum_viewport_width;
 
 	/**
-	 * Maximum possible viewport width for the group (inclusive).
+	 * Maximum possible viewport width for the group (inclusive), where null means it is unbounded.
 	 *
 	 * @since 0.1.0
 	 *
-	 * @var int
-	 * @phpstan-var positive-int
+	 * @var int<1, max>|null
 	 */
 	private $maximum_viewport_width;
 
@@ -56,8 +53,7 @@
 	 *
 	 * @since 0.1.0
 	 *
-	 * @var int
-	 * @phpstan-var positive-int
+	 * @var int<1, max>
 	 */
 	private $sample_size;
 
@@ -66,8 +62,7 @@
 	 *
 	 * @since 0.1.0
 	 *
-	 * @var int
-	 * @phpstan-var 0|positive-int
+	 * @var int<0, max>
 	 */
 	private $freshness_ttl;
 
@@ -99,32 +94,41 @@
 	 *
 	 * This class should never be directly constructed. It should only be constructed by the {@see OD_URL_Metric_Group_Collection::create_groups()}.
 	 *
+	 * @since 0.1.0
+	 *
 	 * @access private
 	 * @throws InvalidArgumentException If arguments are invalid.
 	 *
+	 * @phpstan-param int<0, max>      $minimum_viewport_width
+	 * @phpstan-param int<1, max>|null $maximum_viewport_width
+	 * @phpstan-param int<1, max>      $sample_size
+	 * @phpstan-param int<0, max>      $freshness_ttl
+	 *
 	 * @param OD_URL_Metric[]                $url_metrics            URL Metrics to add to the group.
-	 * @param int                            $minimum_viewport_width Minimum possible viewport width for the group. Must be zero or greater.
-	 * @param int                            $maximum_viewport_width Maximum possible viewport width for the group. Must be greater than zero and the minimum viewport width.
+	 * @param int                            $minimum_viewport_width Minimum possible viewport width (exclusive) for the group. Must be zero or greater.
+	 * @param int|null                       $maximum_viewport_width Maximum possible viewport width (inclusive) for the group. Must be greater than zero and the minimum viewport width. Null means unbounded.
 	 * @param int                            $sample_size            Sample size for the maximum number of viewports in a group between breakpoints.
 	 * @param int                            $freshness_ttl          Freshness age (TTL) for a given URL Metric.
 	 * @param OD_URL_Metric_Group_Collection $collection             Collection that this instance belongs to.
 	 */
-	public function __construct( array $url_metrics, int $minimum_viewport_width, int $maximum_viewport_width, int $sample_size, int $freshness_ttl, OD_URL_Metric_Group_Collection $collection ) {
+	public function __construct( array $url_metrics, int $minimum_viewport_width, ?int $maximum_viewport_width, int $sample_size, int $freshness_ttl, OD_URL_Metric_Group_Collection $collection ) {
 		if ( $minimum_viewport_width < 0 ) {
 			throw new InvalidArgumentException(
 				esc_html__( 'The minimum viewport width must be at least zero.', 'optimization-detective' )
 			);
 		}
-		if ( $maximum_viewport_width < 1 ) {
-			throw new InvalidArgumentException(
-				esc_html__( 'The maximum viewport width must be greater than zero.', 'optimization-detective' )
-			);
+		if ( isset( $maximum_viewport_width ) ) {
+			if ( $maximum_viewport_width < 1 ) {
+				throw new InvalidArgumentException(
+					esc_html__( 'The maximum viewport width must be greater than zero.', 'optimization-detective' )
+				);
+			}
+			if ( $minimum_viewport_width >= $maximum_viewport_width ) {
+				throw new InvalidArgumentException(
+					esc_html__( 'The minimum viewport width must be smaller than the maximum viewport width.', 'optimization-detective' )
+				);
+			}
 		}
-		if ( $minimum_viewport_width >= $maximum_viewport_width ) {
-			throw new InvalidArgumentException(
-				esc_html__( 'The minimum viewport width must be smaller than the maximum viewport width.', 'optimization-detective' )
-			);
-		}
 		$this->minimum_viewport_width = $minimum_viewport_width;
 		$this->maximum_viewport_width = $maximum_viewport_width;
 
@@ -158,12 +162,12 @@
 	}
 
 	/**
-	 * Gets the minimum possible viewport width (inclusive).
+	 * Gets the minimum possible viewport width (exclusive).
 	 *
 	 * @since 0.1.0
 	 *
 	 * @todo Eliminate in favor of readonly public property.
-	 * @return int<0, max> Minimum viewport width.
+	 * @return int<0, max> Minimum viewport width (exclusive).
 	 */
 	public function get_minimum_viewport_width(): int {
 		return $this->minimum_viewport_width;
@@ -175,9 +179,9 @@
 	 * @since 0.1.0
 	 *
 	 * @todo Eliminate in favor of readonly public property.
-	 * @return int<1, max> Minimum viewport width.
+	 * @return int<1, max>|null Minimum viewport width (inclusive). Null means unbounded.
 	 */
-	public function get_maximum_viewport_width(): int {
+	public function get_maximum_viewport_width(): ?int {
 		return $this->maximum_viewport_width;
 	}
 
@@ -187,8 +191,7 @@
 	 * @since 0.9.0
 	 *
 	 * @todo Eliminate in favor of readonly public property.
-	 * @phpstan-return positive-int
-	 * @return int Sample size.
+	 * @return int<1, max> Sample size.
 	 */
 	public function get_sample_size(): int {
 		return $this->sample_size;
@@ -200,8 +203,7 @@
 	 * @since 0.9.0
 	 *
 	 * @todo Eliminate in favor of readonly public property.
-	 * @phpstan-return 0|positive-int
-	 * @return int Freshness age.
+	 * @return int<0, max> Freshness age.
 	 */
 	public function get_freshness_ttl(): int {
 		return $this->freshness_ttl;
@@ -208,17 +210,31 @@
 	}
 
 	/**
-	 * Checks whether the provided viewport width is within the minimum/maximum range for.
+	 * Gets the collection that this group is a part of.
 	 *
+	 * @since 1.0.0
+	 *
+	 * @todo Eliminate in favor of readonly public property.
+	 * @return OD_URL_Metric_Group_Collection Collection.
+	 */
+	public function get_collection(): OD_URL_Metric_Group_Collection {
+		return $this->collection;
+	}
+
+	/**
+	 * Checks whether the provided viewport width is between the minimum (exclusive) and maximum (inclusive).
+	 *
 	 * @since 0.1.0
 	 *
+	 * @phpstan-param int<1, max> $viewport_width
+	 *
 	 * @param int $viewport_width Viewport width.
 	 * @return bool Whether the viewport width is in range.
 	 */
 	public function is_viewport_width_in_range( int $viewport_width ): bool {
 		return (
-			$viewport_width >= $this->minimum_viewport_width &&
-			$viewport_width <= $this->maximum_viewport_width
+			$viewport_width > $this->minimum_viewport_width &&
+			( null === $this->maximum_viewport_width || $viewport_width <= $this->maximum_viewport_width )
 		);
 	}
 
@@ -286,11 +302,6 @@
 					return false;
 				}
 
-				// The ETag is not populated yet, so this is stale. Eventually this will be required.
-				if ( $url_metric->get_etag() === null ) {
-					return false;
-				}
-
 				// The ETag of the URL Metric does not match the current ETag for the collection, so it is stale.
 				if ( ! hash_equals( $url_metric->get_etag(), $this->collection->get_current_etag() ) ) {
 					return false;
@@ -329,7 +340,7 @@
 			/**
 			 * Seen breadcrumbs counts.
 			 *
-			 * @var array<int, string> $seen_breadcrumbs
+			 * @var array<int, non-empty-string> $seen_breadcrumbs
 			 */
 			$seen_breadcrumbs = array();
 
@@ -336,7 +347,7 @@
 			/**
 			 * Breadcrumb counts.
 			 *
-			 * @var array<int, int> $breadcrumb_counts
+			 * @var array<int, non-negative-int> $breadcrumb_counts
 			 */
 			$breadcrumb_counts = array();
 
@@ -489,8 +500,8 @@
 	 * @return array{
 	 *             freshness_ttl: 0|positive-int,
 	 *             sample_size: positive-int,
-	 *             minimum_viewport_width: 0|positive-int,
-	 *             maximum_viewport_width: positive-int,
+	 *             minimum_viewport_width: int<0, max>,
+	 *             maximum_viewport_width: int<1, max>|null,
 	 *             lcp_element: ?OD_Element,
 	 *             complete: bool,
 	 *             url_metrics: OD_URL_Metric[]
Index: class-od-url-metric.php
===================================================================
--- class-od-url-metric.php	(revision 3238281)
+++ class-od-url-metric.php	(working copy)
@@ -16,8 +16,8 @@
  * Representation of the measurements taken from a single client's visit to a specific URL.
  *
  * @phpstan-type ViewportRect array{
- *                                width: int,
- *                                height: int
+ *                                width: positive-int,
+ *                                height: positive-int
  *                            }
  * @phpstan-type DOMRect      array{
  *                                width: float,
@@ -39,7 +39,7 @@
  *                            }
  * @phpstan-type Data         array{
  *                                uuid: non-empty-string,
- *                                etag?: non-empty-string,
+ *                                etag: non-empty-string,
  *                                url: non-empty-string,
  *                                timestamp: float,
  *                                viewport: ViewportRect,
@@ -60,7 +60,6 @@
  *                            }
  *
  * @since 0.1.0
- * @access private
  */
 class OD_URL_Metric implements JsonSerializable {
 
@@ -67,6 +66,7 @@
 	/**
 	 * Data.
 	 *
+	 * @since 0.1.0
 	 * @var Data
 	 */
 	protected $data;
@@ -74,6 +74,7 @@
 	/**
 	 * Elements.
 	 *
+	 * @since 0.7.0
 	 * @var OD_Element[]
 	 */
 	protected $elements;
@@ -89,6 +90,8 @@
 	/**
 	 * Constructor.
 	 *
+	 * @since 0.1.0
+	 *
 	 * @phpstan-param Data|array<string, mixed> $data Valid data or invalid data (in which case an exception is thrown).
 	 *
 	 * @throws OD_Data_Validation_Exception When the input is invalid.
@@ -105,6 +108,8 @@
 	/**
 	 * Prepares data with validation and sanitization.
 	 *
+	 * @since 0.6.0
+	 *
 	 * @throws OD_Data_Validation_Exception When the input is invalid.
 	 *
 	 * @param array<string, mixed> $data Data to validate.
@@ -171,6 +176,7 @@
 	 *
 	 * @since 0.1.0
 	 * @since 0.9.0 Added the 'etag' property to the schema.
+	 * @since 1.0.0 The 'etag' property is now required.
 	 *
 	 * @todo Cache the return value?
 	 *
@@ -230,7 +236,7 @@
 					'pattern'     => '^[0-9a-f]{32}\z',
 					'minLength'   => 32,
 					'maxLength'   => 32,
-					'required'    => false, // To be made required in a future release.
+					'required'    => true,
 					'readonly'    => true, // Omit from REST API.
 				),
 				'url'       => array(
@@ -248,12 +254,12 @@
 						'width'  => array(
 							'type'     => 'integer',
 							'required' => true,
-							'minimum'  => 0,
+							'minimum'  => 1,
 						),
 						'height' => array(
 							'type'     => 'integer',
 							'required' => true,
-							'minimum'  => 0,
+							'minimum'  => 1,
 						),
 					),
 					'additionalProperties' => false,
@@ -349,7 +355,6 @@
 	 * @param array<string, mixed> $properties_schema     Properties schema to extend.
 	 * @param array<string, mixed> $additional_properties Additional properties.
 	 * @param string               $filter_name           Filter name used to extend.
-	 *
 	 * @return array<string, mixed> Extended schema.
 	 */
 	protected static function extend_schema_with_optional_properties( array $properties_schema, array $additional_properties, string $filter_name ): array {
@@ -436,7 +441,7 @@
 	 *
 	 * @since 0.6.0
 	 *
-	 * @return string UUID.
+	 * @return non-empty-string UUID.
 	 */
 	public function get_uuid(): string {
 		return $this->data['uuid'];
@@ -446,12 +451,12 @@
 	 * Gets ETag.
 	 *
 	 * @since 0.9.0
+	 * @since 1.0.0 No longer returns null as 'etag' is now required.
 	 *
-	 * @return non-empty-string|null ETag.
+	 * @return non-empty-string ETag.
 	 */
-	public function get_etag(): ?string {
-		// Since the ETag is optional for now, return null for old URL Metrics that do not have one.
-		return $this->data['etag'] ?? null;
+	public function get_etag(): string {
+		return $this->data['etag'];
 	}
 
 	/**
@@ -459,7 +464,7 @@
 	 *
 	 * @since 0.1.0
 	 *
-	 * @return string URL.
+	 * @return non-empty-string URL.
 	 */
 	public function get_url(): string {
 		return $this->data['url'];
@@ -481,7 +486,7 @@
 	 *
 	 * @since 0.1.0
 	 *
-	 * @return int Viewport width.
+	 * @return positive-int Viewport width.
 	 */
 	public function get_viewport_width(): int {
 		return $this->data['viewport']['width'];
Index: class-od-visited-tag-state.php
===================================================================
--- class-od-visited-tag-state.php	(revision 3238281)
+++ class-od-visited-tag-state.php	(working copy)
@@ -30,6 +30,8 @@
 
 	/**
 	 * Constructor.
+	 *
+	 * @since 1.0.0
 	 */
 	public function __construct() {
 		$this->reset();
Index: detect.js
===================================================================
--- detect.js	(revision 3238281)
+++ detect.js	(working copy)
@@ -96,25 +96,68 @@
 }
 
 /**
- * Checks whether the URL Metric(s) for the provided viewport width is needed.
+ * Gets the status for the URL Metric group for the provided viewport width.
  *
+ * The comparison logic here corresponds with the PHP logic in `OD_URL_Metric_Group::is_viewport_width_in_range()`.
+ * This function is also similar to the PHP logic in `\OD_URL_Metric_Group_Collection::get_group_for_viewport_width()`.
+ *
  * @param {number}                 viewportWidth          - Current viewport width.
  * @param {URLMetricGroupStatus[]} urlMetricGroupStatuses - Viewport group statuses.
- * @return {boolean} Whether URL Metrics are needed.
+ * @return {URLMetricGroupStatus} The URL metric group for the viewport width.
  */
-function isViewportNeeded( viewportWidth, urlMetricGroupStatuses ) {
-	let lastWasLacking = false;
-	for ( const { minimumViewportWidth, complete } of urlMetricGroupStatuses ) {
-		if ( viewportWidth >= minimumViewportWidth ) {
-			lastWasLacking = ! complete;
-		} else {
-			break;
+function getGroupForViewportWidth( viewportWidth, urlMetricGroupStatuses ) {
+	for ( const urlMetricGroupStatus of urlMetricGroupStatuses ) {
+		if (
+			viewportWidth > urlMetricGroupStatus.minimumViewportWidth &&
+			( null === urlMetricGroupStatus.maximumViewportWidth ||
+				viewportWidth <= urlMetricGroupStatus.maximumViewportWidth )
+		) {
+			return urlMetricGroupStatus;
 		}
 	}
-	return lastWasLacking;
+	throw new Error(
+		`${ consoleLogPrefix } Unexpectedly unable to locate group for the current viewport width.`
+	);
 }
 
 /**
+ * Gets the sessionStorage key for keeping track of whether the current client session already submitted a URL Metric.
+ *
+ * @param {string}               currentETag          - Current ETag.
+ * @param {string}               currentUrl           - Current URL.
+ * @param {URLMetricGroupStatus} urlMetricGroupStatus - URL Metric group status.
+ * @return {Promise<string>} Session storage key.
+ */
+async function getAlreadySubmittedSessionStorageKey(
+	currentETag,
+	currentUrl,
+	urlMetricGroupStatus
+) {
+	const message = [
+		currentETag,
+		currentUrl,
+		urlMetricGroupStatus.minimumViewportWidth,
+		urlMetricGroupStatus.maximumViewportWidth || '',
+	].join( '-' );
+
+	/*
+	 * Note that the components are hashed for a couple of reasons:
+	 *
+	 * 1. It results in a consistent length string devoid of any special characters that could cause problems.
+	 * 2. Since the key includes the URL, hashing it avoids potential privacy concerns where the sessionStorage is
+	 *    examined to see which URLs the client went to.
+	 *
+	 * The SHA-1 algorithm is chosen since it is the fastest and there is no need for cryptographic security.
+	 */
+	const msgBuffer = new TextEncoder().encode( message );
+	const hashBuffer = await crypto.subtle.digest( 'SHA-1', msgBuffer );
+	const hashHex = Array.from( new Uint8Array( hashBuffer ) )
+		.map( ( b ) => b.toString( 16 ).padStart( 2, '0' ) )
+		.join( '' );
+	return `odSubmitted-${ hashHex }`;
+}
+
+/**
  * Gets the current time in milliseconds.
  *
  * @return {number} Current time in milliseconds.
@@ -253,6 +296,7 @@
  * @param {number}                 args.maxViewportAspectRatio     Maximum aspect ratio allowed for the viewport.
  * @param {boolean}                args.isDebug                    Whether to show debug messages.
  * @param {string}                 args.restApiEndpoint            URL for where to send the detection data.
+ * @param {string}                 [args.restApiNonce]             Nonce for the REST API when the user is logged-in.
  * @param {string}                 args.currentETag                Current ETag.
  * @param {string}                 args.currentUrl                 Current URL.
  * @param {string}                 args.urlMetricSlug              Slug for URL Metric.
@@ -260,6 +304,7 @@
  * @param {string}                 args.urlMetricHMAC              HMAC for URL Metric storage.
  * @param {URLMetricGroupStatus[]} args.urlMetricGroupStatuses     URL Metric group statuses.
  * @param {number}                 args.storageLockTTL             The TTL (in seconds) for the URL Metric storage lock.
+ * @param {number}                 args.freshnessTTL               The freshness age (TTL) for a given URL Metric.
  * @param {string}                 args.webVitalsLibrarySrc        The URL for the web-vitals library.
  * @param {CollectionDebugData}    [args.urlMetricGroupCollection] URL Metric group collection, when in debug mode.
  */
@@ -269,6 +314,7 @@
 	isDebug,
 	extensionModuleUrls,
 	restApiEndpoint,
+	restApiNonce,
 	currentETag,
 	currentUrl,
 	urlMetricSlug,
@@ -276,6 +322,7 @@
 	urlMetricHMAC,
 	urlMetricGroupStatuses,
 	storageLockTTL,
+	freshnessTTL,
 	webVitalsLibrarySrc,
 	urlMetricGroupCollection,
 } ) {
@@ -297,8 +344,21 @@
 		);
 	}
 
+	if ( win.innerWidth === 0 || win.innerHeight === 0 ) {
+		if ( isDebug ) {
+			log(
+				'Window must have non-zero dimensions for URL Metric collection.'
+			);
+		}
+		return;
+	}
+
 	// Abort if the current viewport is not among those which need URL Metrics.
-	if ( ! isViewportNeeded( win.innerWidth, urlMetricGroupStatuses ) ) {
+	const urlMetricGroupStatus = getGroupForViewportWidth(
+		win.innerWidth,
+		urlMetricGroupStatuses
+	);
+	if ( urlMetricGroupStatus.complete ) {
 		if ( isDebug ) {
 			log( 'No need for URL Metrics from the current viewport.' );
 		}
@@ -305,6 +365,31 @@
 		return;
 	}
 
+	// Abort if the client already submitted a URL Metric for this URL and viewport group.
+	const alreadySubmittedSessionStorageKey =
+		await getAlreadySubmittedSessionStorageKey(
+			currentETag,
+			currentUrl,
+			urlMetricGroupStatus
+		);
+	if ( alreadySubmittedSessionStorageKey in sessionStorage ) {
+		const previousVisitTime = parseInt(
+			sessionStorage.getItem( alreadySubmittedSessionStorageKey ),
+			10
+		);
+		if (
+			! isNaN( previousVisitTime ) &&
+			( getCurrentTime() - previousVisitTime ) / 1000 < freshnessTTL
+		) {
+			if ( isDebug ) {
+				log(
+					'The current client session already submitted a fresh URL Metric for this URL so a new one will not be collected now.'
+				);
+				return;
+			}
+		}
+	}
+
 	// Abort if the viewport aspect ratio is not in a common range.
 	const aspectRatio = win.innerWidth / win.innerHeight;
 	if (
@@ -603,6 +688,7 @@
 		return;
 	}
 
+	// Finalize extensions.
 	if ( extensions.size > 0 ) {
 		/** @type {Promise[]} */
 		const extensionFinalizePromises = [];
@@ -655,15 +741,63 @@
 		}
 	}
 
+	/*
+	 * Now prepare the URL Metric to be sent as JSON request body.
+	 */
+
+	const maxBodyLengthKiB = 64;
+	const maxBodyLengthBytes = maxBodyLengthKiB * 1024;
+
+	// TODO: Consider adding replacer to reduce precision on numbers in DOMRect to reduce payload size.
+	const jsonBody = JSON.stringify( urlMetric );
+	const percentOfBudget =
+		( jsonBody.length / ( maxBodyLengthKiB * 1000 ) ) * 100;
+
+	/*
+	 * According to the fetch() spec:
+	 * "If the sum of contentLength and inflightKeepaliveBytes is greater than 64 kibibytes, then return a network error."
+	 * This is what browsers also implement for navigator.sendBeacon(). Therefore, if the size of the JSON is greater
+	 * than the maximum, we should avoid even trying to send it.
+	 */
+	if ( jsonBody.length > maxBodyLengthBytes ) {
+		if ( isDebug ) {
+			error(
+				`Unable to send URL Metric because it is ${ jsonBody.length.toLocaleString() } bytes, ${ Math.round(
+					percentOfBudget
+				) }% of ${ maxBodyLengthKiB } KiB limit:`,
+				urlMetric
+			);
+		}
+		return;
+	}
+
 	// Even though the server may reject the REST API request, we still have to set the storage lock
 	// because we can't look at the response when sending a beacon.
 	setStorageLock( getCurrentTime() );
 
+	// Remember that the URL Metric was submitted for this URL to avoid having multiple entries submitted by the same client.
+	sessionStorage.setItem(
+		alreadySubmittedSessionStorageKey,
+		String( getCurrentTime() )
+	);
+
 	if ( isDebug ) {
-		log( 'Sending URL Metric:', urlMetric );
+		const message = `Sending URL Metric (${ jsonBody.length.toLocaleString() } bytes, ${ Math.round(
+			percentOfBudget
+		) }% of ${ maxBodyLengthKiB } KiB limit):`;
+
+		// The threshold of 50% is used because the limit for all beacons combined is 64 KiB, not just the data for one beacon.
+		if ( percentOfBudget < 50 ) {
+			log( message, urlMetric );
+		} else {
+			warn( message, urlMetric );
+		}
 	}
 
 	const url = new URL( restApiEndpoint );
+	if ( typeof restApiNonce === 'string' ) {
+		url.searchParams.set( '_wpnonce', restApiNonce );
+	}
 	url.searchParams.set( 'slug', urlMetricSlug );
 	url.searchParams.set( 'current_etag', currentETag );
 	if ( typeof cachePurgePostId === 'number' ) {
@@ -675,7 +809,7 @@
 	url.searchParams.set( 'hmac', urlMetricHMAC );
 	navigator.sendBeacon(
 		url,
-		new Blob( [ JSON.stringify( urlMetric ) ], {
+		new Blob( [ jsonBody ], {
 			type: 'application/json',
 		} )
 	);
Index: detect.min.js
===================================================================
--- detect.min.js	(revision 3238281)
+++ detect.min.js	(working copy)
@@ -1 +1 @@
(Large diff suppressed)

Index: detection.php
===================================================================
--- detection.php	(revision 3238281)
+++ detection.php	(working copy)
@@ -38,11 +38,11 @@
  *
  * @global WP_Query $wp_query WordPress Query object.
  *
- * @return int|null Post ID or null if none found.
+ * @return positive-int|null Post ID or null if none found.
  */
 function od_get_cache_purge_post_id(): ?int {
 	$queried_object = get_queried_object();
-	if ( $queried_object instanceof WP_Post ) {
+	if ( $queried_object instanceof WP_Post && $queried_object->ID > 0 ) {
 		return $queried_object->ID;
 	}
 
@@ -55,6 +55,8 @@
 		isset( $wp_query->posts[0] )
 		&&
 		$wp_query->posts[0] instanceof WP_Post
+		&&
+		$wp_query->posts[0]->ID > 0
 	) {
 		return $wp_query->posts[0]->ID;
 	}
@@ -68,7 +70,7 @@
  * @since 0.1.0
  * @access private
  *
- * @param string                         $slug             URL Metrics slug.
+ * @param non-empty-string               $slug             URL Metrics slug.
  * @param OD_URL_Metric_Group_Collection $group_collection URL Metric group collection.
  */
 function od_get_detection_script( string $slug, OD_URL_Metric_Group_Collection $group_collection ): string {
@@ -128,7 +130,8 @@
 		'urlMetricGroupStatuses' => array_map(
 			static function ( OD_URL_Metric_Group $group ): array {
 				return array(
-					'minimumViewportWidth' => $group->get_minimum_viewport_width(),
+					'minimumViewportWidth' => $group->get_minimum_viewport_width(), // Exclusive.
+					'maximumViewportWidth' => $group->get_maximum_viewport_width(), // Inclusive.
 					'complete'             => $group->is_complete(),
 				);
 			},
@@ -135,8 +138,12 @@
 			iterator_to_array( $group_collection )
 		),
 		'storageLockTTL'         => OD_Storage_Lock::get_ttl(),
+		'freshnessTTL'           => od_get_url_metric_freshness_ttl(),
 		'webVitalsLibrarySrc'    => $web_vitals_lib_src,
 	);
+	if ( is_user_logged_in() ) {
+		$detect_args['restApiNonce'] = wp_create_nonce( 'wp_rest' );
+	}
 	if ( WP_DEBUG ) {
 		$detect_args['urlMetricGroupCollection'] = $group_collection;
 	}
Index: helper.php
===================================================================
--- helper.php	(revision 3238281)
+++ helper.php	(working copy)
@@ -22,6 +22,22 @@
 	/**
 	 * Fires when extensions to Optimization Detective can be loaded and initialized.
 	 *
+	 * This action is useful for loading extension code that depends on Optimization Detective to be running. The version
+	 * of the plugin is passed as the sole argument so that if the required version is not present, the callback can short circuit.
+	 *
+	 * Example:
+	 *
+	 *     add_action( 'od_init', function ( string $version ) {
+	 *         if ( version_compare( $version, '1.0', '<' ) ) {
+	 *             add_action( 'admin_notices', 'my_plugin_warn_optimization_plugin_outdated' );
+	 *             return;
+	 *         }
+	 *
+	 *         // Bootstrap the Optimization Detective extension.
+	 *         require_once __DIR__ . '/functions.php';
+	 *         // ...
+	 *     } );
+	 *
 	 * @since 0.7.0
 	 *
 	 * @param string $version Optimization Detective version.
@@ -37,26 +53,26 @@
  *
  * @since 0.7.0
  *
- * @param int|null $minimum_viewport_width Minimum viewport width.
- * @param int|null $maximum_viewport_width Maximum viewport width.
+ * @param int<0, max>|null $minimum_viewport_width Minimum viewport width (exclusive).
+ * @param int<1, max>|null $maximum_viewport_width Maximum viewport width (inclusive).
  * @return non-empty-string|null Media query, or null if the min/max were both unspecified or invalid.
  */
 function od_generate_media_query( ?int $minimum_viewport_width, ?int $maximum_viewport_width ): ?string {
-	if ( is_int( $minimum_viewport_width ) && is_int( $maximum_viewport_width ) && $minimum_viewport_width > $maximum_viewport_width ) {
-		_doing_it_wrong( __FUNCTION__, esc_html__( 'The minimum width cannot be greater than the maximum width.', 'optimization-detective' ), 'Optimization Detective 0.7.0' );
+	if ( is_int( $minimum_viewport_width ) && is_int( $maximum_viewport_width ) && $minimum_viewport_width >= $maximum_viewport_width ) {
+		_doing_it_wrong( __FUNCTION__, esc_html__( 'The minimum width cannot be greater than or equal to the maximum width.', 'optimization-detective' ), 'Optimization Detective 0.7.0' );
 		return null;
 	}
-	$media_attributes = array();
-	if ( null !== $minimum_viewport_width && $minimum_viewport_width > 0 ) {
-		$media_attributes[] = sprintf( '(min-width: %dpx)', $minimum_viewport_width );
-	}
-	if ( null !== $maximum_viewport_width && PHP_INT_MAX !== $maximum_viewport_width ) {
-		$media_attributes[] = sprintf( '(max-width: %dpx)', $maximum_viewport_width );
-	}
-	if ( count( $media_attributes ) === 0 ) {
+	$has_min_width = ( null !== $minimum_viewport_width && $minimum_viewport_width > 0 );
+	$has_max_width = ( null !== $maximum_viewport_width && PHP_INT_MAX !== $maximum_viewport_width ); // Note: The use of PHP_INT_MAX is obsolete.
+	if ( $has_min_width && $has_max_width ) {
+		return sprintf( '(%dpx < width <= %dpx)', $minimum_viewport_width, $maximum_viewport_width );
+	} elseif ( $has_min_width ) {
+		return sprintf( '(%dpx < width)', $minimum_viewport_width );
+	} elseif ( $has_max_width ) {
+		return sprintf( '(width <= %dpx)', $maximum_viewport_width );
+	} else {
 		return null;
 	}
-	return join( ' and ', $media_attributes );
 }
 
 /**
Index: hooks.php
===================================================================
--- hooks.php	(revision 3238281)
+++ hooks.php	(working copy)
@@ -18,6 +18,7 @@
 add_action( 'init', 'od_initialize_extensions', PHP_INT_MAX );
 add_filter( 'template_include', 'od_buffer_output', PHP_INT_MAX );
 OD_URL_Metrics_Post_Type::add_hooks();
+OD_Storage_Lock::add_hooks();
 add_action( 'wp', 'od_maybe_add_template_output_buffer_filter' );
 add_action( 'wp_head', 'od_render_generator_meta_tag' );
 add_filter( 'site_status_tests', 'od_add_rest_api_availability_test' );
Index: load.php
===================================================================
--- load.php	(revision 3238281)
+++ load.php	(working copy)
@@ -2,10 +2,10 @@
 /**
  * Plugin Name: Optimization Detective
  * Plugin URI: https://github.com/WordPress/performance/tree/trunk/plugins/optimization-detective
- * Description: Provides an API for leveraging real user metrics to detect optimizations to apply on the frontend to improve page performance.
+ * Description: Provides a framework for leveraging real user metrics to detect optimizations for improving page performance.
  * Requires at least: 6.6
  * Requires PHP: 7.2
- * Version: 1.0.0-beta1
+ * Version: 1.0.0-beta2
  * Author: WordPress Performance Team
  * Author URI: https://make.wordpress.org/performance/
  * License: GPLv2 or later
@@ -71,7 +71,7 @@
 	}
 )(
 	'optimization_detective_pending_plugin',
-	'1.0.0-beta1',
+	'1.0.0-beta2',
 	static function ( string $version ): void {
 		if ( defined( 'OPTIMIZATION_DETECTIVE_VERSION' ) ) {
 			return;
Index: optimization.php
===================================================================
--- optimization.php	(revision 3238281)
+++ optimization.php	(working copy)
@@ -167,6 +167,8 @@
 		// > Access to script at '.../detect.js?ver=0.4.1' from origin 'null' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.
 		// So it's better to just avoid attempting to optimize Post Embed responses (which don't need optimization anyway).
 		is_embed() ||
+		// Skip posts that aren't published yet.
+		is_preview() ||
 		// Since injection of inline-editing controls interfere with breadcrumbs, while also just not necessary in this context.
 		is_customize_preview() ||
 		// Since the images detected in the response body of a POST request cannot, by definition, be cached.
@@ -251,6 +253,77 @@
 	/**
 	 * Fires to register tag visitors before walking over the document to perform optimizations.
 	 *
+	 * Once a page has finished rendering and the output buffer is processed, the page contents are loaded into
+	 * an HTML Tag Processor instance. It then iterates over each tag in the document, and at each open tag it will
+	 * invoke all registered tag visitors. A tag visitor is simply a callable (such as a regular function, closure,
+	 * or even a class with an `__invoke` method defined). The tag visitor callback is invoked by passing an instance
+	 * of the `OD_Tag_Visitor_Context` object which includes the following read-only properties:
+	 *
+	 * - `$processor` (`OD_HTML_Tag_Processor`): The processor with the cursor at the current open tag.
+	 * - `$url_metric_group_collection` (`OD_URL_Metric_Group_Collection`): The URL Metrics which may include information about the current tag to inform what optimizations the callback performs.
+	 * - `$link_collection` (`OD_Link_Collection`): Collection of links which will be added to the `HEAD` when the page is served. This allows you to add preload links and preconnect links as needed.
+	 * - `$url_metrics_id` (`positive-int|null`): The post ID for the `od_url_metrics` post from which the URL Metrics were loaded (if any). For advanced usage.
+	 *
+	 * Note that you are free to call `$processor->next_tag()` in the callback (such as to walk over any child elements)
+	 * since the tag processor's cursor will be reset to the tag after the callback finishes.
+	 *
+	 * When a tag visitor sees it is at a relevant open tag (e.g. by checking `$processor->get_tag()`), it can call the
+	 * `$context->track_tag()` method to indicate that the tag should be measured during detection. This will cause the
+	 * tag to be included among the `elements` in the stored URL Metrics. The element data includes properties such
+	 * as `intersectionRatio`, `intersectionRect`, and `boundingClientRect` (provided by an `IntersectionObserver`) as
+	 * well as whether the tag is the LCP element (`isLCP`) or LCP element candidate (`isLCPCandidate`). This method
+	 * should not be called if the current tag is not relevant for the tag visitor or if the tag visitor callback does
+	 * not need to query the provided `OD_URL_Metric_Group_Collection` instance to apply the desired optimizations. (In
+	 * addition to calling the `$context->track_tag()`, a callback may also return `true` to indicate the tag should be
+	 * tracked.)
+	 *
+	 * Here's an example tag visitor that depends on URL Metrics data:
+	 *
+	 *     $tag_visitor_registry->register(
+	 *         'lcp-img-fetchpriority-high',
+	 *         static function ( OD_Tag_Visitor_Context $context ): void {
+	 *             if ( $context->processor->get_tag() !== 'IMG' ) {
+	 *                 return; // Tag is not relevant for this tag visitor.
+	 *             }
+	 *
+	 *             // Mark the tag for measurement during detection so it is included among the elements stored in URL Metrics.
+	 *             $context->track_tag();
+	 *
+	 *             // Make sure fetchpriority=high is added to LCP IMG elements based on the captured URL Metrics.
+	 *             $common_lcp_element = $context->url_metric_group_collection->get_common_lcp_element();
+	 *             if (
+	 *                 null !== $common_lcp_element
+	 *                 &&
+	 *                 $common_lcp_element->get_xpath() === $context->processor->get_xpath()
+	 *             ) {
+	 *                 $context->processor->set_attribute( 'fetchpriority', 'high' );
+	 *             }
+	 *         }
+	 *     );
+	 *
+	 * Please note this implementation of setting `fetchpriority=high` on the LCP `IMG` element is simplified. Please
+	 * see the Image Prioritizer extension for a more robust implementation.
+	 *
+	 * Here's an example tag visitor that does not depend on any URL Metrics data:
+	 *
+	 *     $tag_visitor_registry->register(
+	 *         'img-decoding-async',
+	 *         static function ( OD_Tag_Visitor_Context $context ): bool {
+	 *             if ( $context->processor->get_tag() !== 'IMG' ) {
+	 *                 return; // Tag is not relevant for this tag visitor.
+	 *             }
+	 *
+	 *             // Set the decoding attribute if it is absent.
+	 *             if ( null === $context->processor->get_attribute( 'decoding' ) ) {
+	 *                 $context->processor->set_attribute( 'decoding', 'async' );
+	 *             }
+	 *         }
+	 *     );
+	 *
+	 * Refer to [Image Prioritizer](https://github.com/WordPress/performance/tree/trunk/plugins/image-prioritizer) and
+	 * [Embed Optimizer](https://github.com/WordPress/performance/tree/trunk/plugins/embed-optimizer) for additional
+	 * examples of how tag visitors are used.
+	 *
 	 * @since 0.3.0
 	 *
 	 * @param OD_Tag_Visitor_Registry $tag_visitor_registry Tag visitor registry.
@@ -267,7 +340,13 @@
 	);
 	$link_collection      = new OD_Link_Collection();
 	$visited_tag_state    = new OD_Visited_Tag_State();
-	$tag_visitor_context  = new OD_Tag_Visitor_Context( $processor, $group_collection, $link_collection, $visited_tag_state );
+	$tag_visitor_context  = new OD_Tag_Visitor_Context(
+		$processor,
+		$group_collection,
+		$link_collection,
+		$visited_tag_state,
+		$post instanceof WP_Post && $post->ID > 0 ? $post->ID : null
+	);
 	$current_tag_bookmark = 'optimization_detective_current_tag';
 	$visitors             = iterator_to_array( $tag_visitor_registry );
 
Index: readme.txt
===================================================================
--- readme.txt	(revision 3238281)
+++ readme.txt	(working copy)
@@ -2,12 +2,12 @@
 
 Contributors: wordpressdotorg
 Tested up to: 6.7
-Stable tag:   1.0.0-beta1
+Stable tag:   1.0.0-beta2
 License:      GPLv2 or later
 License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 Tags:         performance, optimization, rum
 
-Provides an API for leveraging real user metrics to detect optimizations to apply on the frontend to improve page performance.
+Provides a framework for leveraging real user metrics to detect optimizations for improving page performance.
 
 == Description ==
 
@@ -55,6 +55,28 @@
 
 == Changelog ==
 
+= 1.0.0-beta2 =
+
+**Enhancements**
+
+* Account for 64 KiB limit for sending beacon data. ([1851](https://github.com/WordPress/performance/pull/1851))
+* Add post ID for the `od_url_metrics` post to the tag visitor context. ([1847](https://github.com/WordPress/performance/pull/1847))
+* Change minimum viewport width to be exclusive whereas the maximum width remains inclusive. ([1839](https://github.com/WordPress/performance/pull/1839))
+* Disable URL Metric storage locking by default for administrators. ([1835](https://github.com/WordPress/performance/pull/1835))
+* Include active plugins in ETag data and increase default freshness TTL from 1 day to 1 week. ([1854](https://github.com/WordPress/performance/pull/1854))
+* Make ETag a required property of the URL Metric. ([1824](https://github.com/WordPress/performance/pull/1824))
+* Use CSS range syntax in media queries. ([1833](https://github.com/WordPress/performance/pull/1833))
+* Use `IFRAME` to display HTML responses for REST API storage request failures in Site Health test. ([1849](https://github.com/WordPress/performance/pull/1849))
+
+**Bug Fixes**
+
+* Prevent URL in `Link` header from including invalid characters. ([1802](https://github.com/WordPress/performance/pull/1802))
+* Prevent optimizing post previews by default. ([1848](https://github.com/WordPress/performance/pull/1848))
+
+**Documentation**
+
+* Improve Optimization Detective documentation. ([1782](https://github.com/WordPress/performance/pull/1782))
+
 = 1.0.0-beta1 =
 
 **Enhancements**
Index: site-health.php
===================================================================
--- site-health.php	(revision 3238281)
+++ site-health.php	(working copy)
@@ -127,6 +127,10 @@
 		$message = wp_remote_retrieve_response_message( $response );
 		$body    = wp_remote_retrieve_body( $response );
 		$data    = json_decode( $body, true );
+		$header  = wp_remote_retrieve_header( $response, 'content-type' );
+		if ( is_array( $header ) ) {
+			$header = array_pop( $header );
+		}
 
 		$is_expected = (
 			400 === $code &&
@@ -156,7 +160,18 @@
 				$result['description'] .= '<blockquote>' . esc_html( $data['message'] ) . '</blockquote>';
 			}
 
-			$result['description'] .= '<details><summary>' . esc_html__( 'Raw response:', 'optimization-detective' ) . '</summary><pre style="white-space: pre-wrap">' . esc_html( $body ) . '</pre></details>';
+			if ( '' !== $body ) {
+				$result['description'] .= '<details>';
+				$result['description'] .= '<summary>' . esc_html__( 'Raw response:', 'optimization-detective' ) . '</summary>';
+
+				if ( is_string( $header ) && str_contains( $header, 'html' ) ) {
+					$escaped_content        = htmlspecialchars( $body, ENT_QUOTES, 'UTF-8' );
+					$result['description'] .= '<iframe srcdoc="' . $escaped_content . '" sandbox width="100%" height="300"></iframe>';
+				} else {
+					$result['description'] .= '<pre style="white-space: pre-wrap">' . esc_html( $body ) . '</pre>';
+				}
+				$result['description'] .= '</details>';
+			}
 		}
 	}
 	return $result;
@@ -238,7 +253,7 @@
 		$message = "<details>$message</details>";
 	}
 
-	wp_admin_notice(
+	$notice = wp_get_admin_notice(
 		$message,
 		array(
 			'type'               => 'warning',
@@ -246,6 +261,16 @@
 			'paragraph_wrap'     => false,
 		)
 	);
+
+	echo wp_kses(
+		$notice,
+		array_merge(
+			wp_kses_allowed_html( 'post' ),
+			array(
+				'iframe' => array_fill_keys( array( 'srcdoc', 'sandbox', 'width', 'height' ), true ),
+			)
+		)
+	);
 }
 
 /**
@@ -254,7 +279,7 @@
  * @since 1.0.0
  * @access private
  *
- * @param string $plugin_file Plugin file.
+ * @param non-empty-string $plugin_file Plugin file.
  */
 function od_render_rest_api_health_check_admin_notice_in_plugin_row( string $plugin_file ): void {
 	if ( 'optimization-detective/load.php' !== $plugin_file ) { // TODO: What if a different plugin slug is used?
Index: storage/class-od-storage-lock.php
===================================================================
--- storage/class-od-storage-lock.php	(revision 3238281)
+++ storage/class-od-storage-lock.php	(working copy)
@@ -21,19 +21,54 @@
 final class OD_Storage_Lock {
 
 	/**
+	 * Capability for being able to store a URL Metric now.
+	 *
+	 * @since 1.0.0
+	 * @var string
+	 */
+	const STORE_URL_METRIC_NOW_CAPABILITY = 'od_store_url_metric_now';
+
+	/**
+	 * Adds hooks.
+	 *
+	 * @since 1.0.0
+	 */
+	public static function add_hooks(): void {
+		add_filter( 'user_has_cap', array( __CLASS__, 'filter_user_has_cap' ) );
+	}
+
+	/**
+	 * Filters `user_has_cap` to grant the `od_store_url_metric_now` capability to users who can `manage_options` by default.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @param array<string, bool>|mixed $allcaps Capability names mapped to boolean values for whether the user has that capability.
+	 * @return array<string, bool> Capability names mapped to boolean values for whether the user has that capability.
+	 */
+	public static function filter_user_has_cap( $allcaps ): array {
+		if ( ! is_array( $allcaps ) ) {
+			$allcaps = array();
+		}
+		if ( isset( $allcaps['manage_options'] ) ) {
+			$allcaps['od_store_url_metric_now'] = $allcaps['manage_options'];
+		}
+		return $allcaps;
+	}
+
+	/**
 	 * Gets the TTL (in seconds) for the URL Metric storage lock.
 	 *
 	 * @since 0.1.0
-	 * @access private
 	 *
-	 * @return int TTL in seconds, greater than or equal to zero. A value of zero means that the storage lock should be disabled and thus that transients must not be used.
+	 * @return int<0, max> TTL in seconds, greater than or equal to zero. A value of zero means that the storage lock should be disabled and thus that transients must not be used.
 	 */
 	public static function get_ttl(): int {
+		$ttl = current_user_can( self::STORE_URL_METRIC_NOW_CAPABILITY ) ? 0 : MINUTE_IN_SECONDS;
 
 		/**
-		 * Filters how long a given IP is locked from submitting another metric-storage REST API request.
+		 * Filters how long the current IP is locked from submitting another URL metric storage REST API request.
 		 *
-		 * Filtering the TTL to zero will disable any metric storage locking. This is useful, for example, to disable
+		 * Filtering the TTL to zero will disable any URL Metric storage locking. This is useful, for example, to disable
 		 * locking when a user is logged-in with code like the following:
 		 *
 		 *     add_filter( 'od_metrics_storage_lock_ttl', static function ( int $ttl ): int {
@@ -40,11 +75,16 @@
 		 *         return is_user_logged_in() ? 0 : $ttl;
 		 *     } );
 		 *
+		 * By default, the TTL is zero (0) for authorized users and sixty (60) for everyone else. Whether the current
+		 * user is authorized is determined by whether the user has the `od_store_url_metric_now` capability. This
+		 * custom capability by default maps to the `manage_options` primitive capability via the `user_has_cap` filter.
+		 *
 		 * @since 0.1.0
+		 * @since 1.0.0 This now defaults to zero (0) for authorized users.
 		 *
-		 * @param int $ttl TTL.
+		 * @param int $ttl TTL. Defaults to 60, except zero (0) for authorized users.
 		 */
-		$ttl = (int) apply_filters( 'od_url_metric_storage_lock_ttl', MINUTE_IN_SECONDS );
+		$ttl = (int) apply_filters( 'od_url_metric_storage_lock_ttl', $ttl );
 		return max( 0, $ttl );
 	}
 
@@ -51,8 +91,9 @@
 	/**
 	 * Gets transient key for locking URL Metric storage (for the current IP).
 	 *
-	 * @todo Should the URL be included in the key? Or should a user only be allowed to store one metric?
-	 * @return string Transient key.
+	 * @since 0.1.0
+	 *
+	 * @return non-empty-string Transient key.
 	 */
 	public static function get_transient_key(): string {
 		$ip_address = $_SERVER['HTTP_X_FORWARDED_FOR'] ?? $_SERVER['REMOTE_ADDR'];
@@ -66,7 +107,6 @@
 	 * seconds. Otherwise, if the current TTL is zero, then any transient is deleted.
 	 *
 	 * @since 0.1.0
-	 * @access private
 	 */
 	public static function set_lock(): void {
 		$ttl = self::get_ttl();
@@ -82,7 +122,6 @@
 	 * Checks whether URL Metric storage is locked (for the current IP).
 	 *
 	 * @since 0.1.0
-	 * @access private
 	 *
 	 * @return bool Whether locked.
 	 */
Index: storage/class-od-url-metric-store-request-context.php
===================================================================
--- storage/class-od-url-metric-store-request-context.php	(revision 3238281)
+++ storage/class-od-url-metric-store-request-context.php	(working copy)
@@ -16,7 +16,13 @@
  * Context for when a URL Metric is successfully stored via the REST API.
  *
  * @since 0.7.0
- * @access private
+ *
+ * @property-read WP_REST_Request<array<string, mixed>> $request                     Request.
+ * @property-read positive-int                          $url_metrics_id              ID for the od_url_metrics post.
+ * @property-read OD_URL_Metric_Group_Collection        $url_metric_group_collection URL Metric group collection.
+ * @property-read OD_URL_Metric_Group                   $url_metric_group            URL Metric group.
+ * @property-read OD_URL_Metric                         $url_metric                  URL Metric.
+ * @property-read positive-int                          $post_id                     Deprecated alias for the $url_metrics_id property.
  */
 final class OD_URL_Metric_Store_Request_Context {
 
@@ -23,59 +29,111 @@
 	/**
 	 * Request.
 	 *
+	 * @since 0.7.0
 	 * @var WP_REST_Request<array<string, mixed>>
-	 * @readonly
 	 */
-	public $request;
+	private $request;
 
 	/**
-	 * ID for the URL Metric post.
+	 * ID for the od_url_metrics post.
 	 *
-	 * @var int
-	 * @readonly
+	 * This was originally $post_id which was introduced in 0.7.0.
+	 *
+	 * @since 1.0.0
+	 * @var positive-int
 	 */
-	public $post_id;
+	private $url_metrics_id;
 
 	/**
 	 * URL Metric group collection.
 	 *
+	 * @since 0.7.0
 	 * @var OD_URL_Metric_Group_Collection
-	 * @readonly
 	 */
-	public $url_metric_group_collection;
+	private $url_metric_group_collection;
 
 	/**
 	 * URL Metric group.
 	 *
+	 * @since 0.7.0
 	 * @var OD_URL_Metric_Group
-	 * @readonly
 	 */
-	public $url_metric_group;
+	private $url_metric_group;
 
 	/**
 	 * URL Metric.
 	 *
+	 * @since 0.7.0
 	 * @var OD_URL_Metric
-	 * @readonly
 	 */
-	public $url_metric;
+	private $url_metric;
 
 	/**
 	 * Constructor.
 	 *
+	 * @since 0.7.0
+	 *
 	 * @phpstan-param WP_REST_Request<array<string, mixed>> $request
 	 *
 	 * @param WP_REST_Request                $request                     REST API request.
-	 * @param int                            $post_id                     ID for the URL Metric post.
+	 * @param positive-int                   $url_metrics_id              ID for the URL Metric post.
 	 * @param OD_URL_Metric_Group_Collection $url_metric_group_collection URL Metric group collection.
 	 * @param OD_URL_Metric_Group            $url_metric_group            URL Metric group.
 	 * @param OD_URL_Metric                  $url_metric                  URL Metric.
 	 */
-	public function __construct( WP_REST_Request $request, int $post_id, OD_URL_Metric_Group_Collection $url_metric_group_collection, OD_URL_Metric_Group $url_metric_group, OD_URL_Metric $url_metric ) {
+	public function __construct( WP_REST_Request $request, int $url_metrics_id, OD_URL_Metric_Group_Collection $url_metric_group_collection, OD_URL_Metric_Group $url_metric_group, OD_URL_Metric $url_metric ) {
 		$this->request                     = $request;
-		$this->post_id                     = $post_id;
+		$this->url_metrics_id              = $url_metrics_id;
 		$this->url_metric_group_collection = $url_metric_group_collection;
 		$this->url_metric_group            = $url_metric_group;
 		$this->url_metric                  = $url_metric;
 	}
+
+	/**
+	 * Gets a property.
+	 *
+	 * @since 1.0.0
+	 *
+	 * @param string $name Property name.
+	 * @return mixed Property value.
+	 *
+	 * @throws Error When property is unknown.
+	 */
+	public function __get( string $name ) {
+		switch ( $name ) {
+			case 'request':
+				return $this->request;
+			case 'url_metrics_id':
+				return $this->url_metrics_id;
+			case 'url_metric_group_collection':
+				return $this->url_metric_group_collection;
+			case 'url_metric_group':
+				return $this->url_metric_group;
+			case 'url_metric':
+				return $this->url_metric;
+			case 'post_id':
+				_doing_it_wrong(
+					esc_html( __CLASS__ . '::$' . $name ),
+					esc_html(
+						sprintf(
+							/* translators: %s is class member variable name */
+							__( 'Use %s instead.', 'optimization-detective' ),
+							__CLASS__ . '::$url_metrics_id'
+						)
+					),
+					'optimization-detective 1.0.0'
+				);
+				return $this->url_metrics_id;
+			default:
+				throw new Error(
+					esc_html(
+						sprintf(
+							/* translators: %s is class member variable name */
+							__( 'Unknown property %s.', 'optimization-detective' ),
+							__CLASS__ . '::$' . $name
+						)
+					)
+				);
+		}
+	}
 }
Index: storage/class-od-url-metrics-post-type.php
===================================================================
--- storage/class-od-url-metrics-post-type.php	(revision 3238281)
+++ storage/class-od-url-metrics-post-type.php	(working copy)
@@ -23,6 +23,7 @@
 	/**
 	 * Post type slug.
 	 *
+	 * @since 0.1.0
 	 * @var string
 	 */
 	const SLUG = 'od_url_metrics';
@@ -30,6 +31,7 @@
 	/**
 	 * Event name (hook) for garbage collection of stale URL Metrics posts.
 	 *
+	 * @since 0.1.0
 	 * @var string
 	 */
 	const GC_CRON_EVENT_NAME = 'od_url_metrics_gc';
@@ -37,6 +39,7 @@
 	/**
 	 * Recurrence for garbage collection of stale URL Metrics posts.
 	 *
+	 * @since 0.1.0
 	 * @var string
 	 */
 	const GC_CRON_RECURRENCE = 'daily';
@@ -84,7 +87,7 @@
 	 *
 	 * @since 0.1.0
 	 *
-	 * @param string $slug URL Metrics slug.
+	 * @param non-empty-string $slug URL Metrics slug.
 	 * @return WP_Post|null Post object if exists.
 	 */
 	public static function get_post( string $slug ): ?WP_Post {
@@ -202,9 +205,9 @@
 	 * @since 0.1.0
 	 * @todo There is duplicate logic here with od_handle_rest_request().
 	 *
-	 * @param string        $slug           Slug (hash of normalized query vars).
-	 * @param OD_URL_Metric $new_url_metric New URL Metric.
-	 * @return int|WP_Error Post ID or WP_Error otherwise.
+	 * @param non-empty-string $slug Slug (hash of normalized query vars).
+	 * @param OD_URL_Metric    $new_url_metric New URL Metric.
+	 * @return positive-int|WP_Error Post ID or WP_Error otherwise.
 	 */
 	public static function store_url_metric( string $slug, OD_URL_Metric $new_url_metric ) {
 		$post_data = array(
@@ -226,13 +229,6 @@
 		}
 
 		$etag = $new_url_metric->get_etag();
-		if ( null === $etag ) {
-			// This case actually will never occur in practice because the store_url_metric function is only called
-			// in the REST API endpoint where the ETag parameter is required. It is here exclusively for the sake of
-			// PHPStan's static analysis. This entire condition can be removed in a future release when the 'etag'
-			// property becomes required.
-			return new WP_Error( 'missing_etag' );
-		}
 
 		$group_collection = new OD_URL_Metric_Group_Collection(
 			$url_metrics,
@@ -250,13 +246,8 @@
 		}
 
 		$post_data['post_content'] = wp_json_encode(
-			array_map(
-				static function ( OD_URL_Metric $url_metric ): array {
-					return $url_metric->jsonSerialize();
-				},
-				$group_collection->get_flattened_url_metrics()
-			),
-			JSON_UNESCAPED_SLASHES // No need for escaped slashes since not printed to frontend.
+			$group_collection->get_flattened_url_metrics(),
+			JSON_UNESCAPED_SLASHES // No need for escaping slashes since this JSON is not embedded in HTML.
 		);
 		if ( ! is_string( $post_data['post_content'] ) ) {
 			return new WP_Error( 'json_encode_error', json_last_error_msg() );
Index: storage/data.php
===================================================================
--- storage/data.php	(revision 3238281)
+++ storage/data.php	(working copy)
@@ -20,7 +20,7 @@
  * @since 0.1.0
  * @access private
  *
- * @return int Expiration TTL in seconds.
+ * @return int<0, max> Expiration TTL in seconds.
  */
 function od_get_url_metric_freshness_ttl(): int {
 	/**
@@ -31,9 +31,9 @@
 	 *
 	 * @since 0.1.0
 	 *
-	 * @param int $ttl Expiration TTL in seconds. Defaults to 1 day.
+	 * @param int $ttl Expiration TTL in seconds. Defaults to 1 week.
 	 */
-	$freshness_ttl = (int) apply_filters( 'od_url_metric_freshness_ttl', DAY_IN_SECONDS );
+	$freshness_ttl = (int) apply_filters( 'od_url_metric_freshness_ttl', WEEK_IN_SECONDS );
 
 	if ( $freshness_ttl < 0 ) {
 		_doing_it_wrong(
@@ -58,8 +58,6 @@
  *
  * This is used as a cache key for stored URL Metrics.
  *
- * TODO: For non-singular requests, consider adding the post IDs from The Loop to ensure publishing a new post will invalidate the cache.
- *
  * @since 0.1.0
  * @access private
  *
@@ -117,6 +115,8 @@
 		// phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized
 		$current_url .= ltrim( wp_unslash( $_SERVER['REQUEST_URI'] ), '/' );
 	}
+
+	// TODO: We should be able to assert that this returns an non-empty-string.
 	return esc_url_raw( $current_url );
 }
 
@@ -131,7 +131,7 @@
  * @see od_get_normalized_query_vars()
  *
  * @param array<string, mixed> $query_vars Normalized query vars.
- * @return string Slug.
+ * @return non-empty-string Slug.
  */
 function od_get_url_metrics_slug( array $query_vars ): string {
 	return md5( (string) wp_json_encode( $query_vars ) );
@@ -199,6 +199,17 @@
 		$queried_object_data['type'] = $queried_object->name;
 	}
 
+	$active_plugins = (array) get_option( 'active_plugins', array() );
+	if ( is_multisite() ) {
+		$active_plugins = array_unique(
+			array_merge(
+				$active_plugins,
+				array_keys( (array) get_site_option( 'active_sitewide_plugins', array() ) )
+			)
+		);
+	}
+	sort( $active_plugins );
+
 	$data = array(
 		'xpath_version'    => 2, // Bump whenever a major change to the XPath format occurs so that new URL Metrics are proactively gathered.
 		'tag_visitors'     => array_keys( iterator_to_array( $tag_visitor_registry ) ),
@@ -230,6 +241,7 @@
 				'version' => wp_get_theme()->get( 'Version' ),
 			),
 		),
+		'active_plugins'   => $active_plugins,
 		'current_template' => $current_template instanceof WP_Block_Template ? get_object_vars( $current_template ) : $current_template,
 	);
 
@@ -257,15 +269,22 @@
  * @see od_verify_url_metrics_storage_hmac()
  * @see od_get_url_metrics_slug()
  *
- * @param string           $slug                Slug (hash of normalized query vars).
- * @param non-empty-string $current_etag        Current ETag.
- * @param string           $url                 URL.
- * @param int|null         $cache_purge_post_id Cache purge post ID.
- * @return string HMAC.
+ * @param non-empty-string  $slug                Slug (hash of normalized query vars).
+ * @param non-empty-string  $current_etag        Current ETag.
+ * @param string            $url                 URL.
+ * @param positive-int|null $cache_purge_post_id Cache purge post ID.
+ * @return non-empty-string HMAC.
  */
 function od_get_url_metrics_storage_hmac( string $slug, string $current_etag, string $url, ?int $cache_purge_post_id = null ): string {
 	$action = "store_url_metric:$slug:$current_etag:$url:$cache_purge_post_id";
-	return wp_hash( $action, 'nonce' );
+
+	/**
+	 * HMAC.
+	 *
+	 * @var non-empty-string $hmac
+	 */
+	$hmac = wp_hash( $action, 'nonce' );
+	return $hmac;
 }
 
 /**
@@ -278,11 +297,11 @@
  * @see od_get_url_metrics_storage_hmac()
  * @see od_get_url_metrics_slug()
  *
- * @param string           $hmac                HMAC.
- * @param string           $slug                Slug (hash of normalized query vars).
- * @param non-empty-string $current_etag        Current ETag.
- * @param string           $url                 URL.
- * @param int|null         $cache_purge_post_id Cache purge post ID.
+ * @param non-empty-string  $hmac                HMAC.
+ * @param non-empty-string  $slug                Slug (hash of normalized query vars).
+ * @param non-empty-string  $current_etag        Current ETag.
+ * @param string            $url                 URL.
+ * @param positive-int|null $cache_purge_post_id Cache purge post ID.
  * @return bool Whether the HMAC is valid.
  */
 function od_verify_url_metrics_storage_hmac( string $hmac, string $slug, string $current_etag, string $url, ?int $cache_purge_post_id = null ): bool {
@@ -360,7 +379,7 @@
  * @access private
  * @link https://github.com/WordPress/gutenberg/blob/093d52cbfd3e2c140843d3fb91ad3d03330320a5/packages/base-styles/_breakpoints.scss#L11-L13
  *
- * @return int[] Breakpoint max widths, sorted in ascending order.
+ * @return positive-int[] Breakpoint max widths, sorted in ascending order.
  */
 function od_get_breakpoint_max_widths(): array {
 	$function_name = __FUNCTION__;
@@ -368,20 +387,7 @@
 	$breakpoint_max_widths = array_map(
 		static function ( $original_breakpoint ) use ( $function_name ): int {
 			$breakpoint = $original_breakpoint;
-			if ( PHP_INT_MAX === $breakpoint ) {
-				$breakpoint = PHP_INT_MAX - 1;
-				_doing_it_wrong(
-					esc_html( $function_name ),
-					esc_html(
-						sprintf(
-							/* translators: %s is the actual breakpoint max width */
-							__( 'Breakpoint must be less than PHP_INT_MAX, but saw "%s".', 'optimization-detective' ),
-							$original_breakpoint
-						)
-					),
-					''
-				);
-			} elseif ( $breakpoint <= 0 ) {
+			if ( $breakpoint <= 0 ) {
 				$breakpoint = 1;
 				_doing_it_wrong(
 					esc_html( $function_name ),
@@ -400,12 +406,12 @@
 		/**
 		 * Filters the breakpoint max widths to group URL Metrics for various viewports.
 		 *
-		 * A breakpoint must be greater than zero and less than PHP_INT_MAX. This array may be empty in which case there
+		 * A breakpoint must be greater than zero. This array may be empty in which case there
 		 * are no responsive breakpoints and all URL Metrics are collected in a single group.
 		 *
 		 * @since 0.1.0
 		 *
-		 * @param int[] $breakpoint_max_widths Max widths for viewport breakpoints. Defaults to [480, 600, 782].
+		 * @param positive-int[] $breakpoint_max_widths Max widths for viewport breakpoints. Defaults to [480, 600, 782].
 		 */
 		array_map( 'intval', (array) apply_filters( 'od_breakpoint_max_widths', array( 480, 600, 782 ) ) )
 	);
@@ -425,7 +431,7 @@
  * @since 0.1.0
  * @access private
  *
- * @return int Sample size.
+ * @return int<1, max> Sample size.
  */
 function od_get_url_metrics_breakpoint_sample_size(): int {
 	/**
Index: storage/rest-api.php
===================================================================
--- storage/rest-api.php	(revision 3238281)
+++ storage/rest-api.php	(working copy)
@@ -15,6 +15,8 @@
 /**
  * Namespace for optimization-detective.
  *
+ * @since 0.1.0
+ * @access private
  * @var string
  */
 const OD_REST_API_NAMESPACE = 'optimization-detective/v1';
@@ -26,6 +28,8 @@
  * that does not strictly follow the standard usage. Namely, submitting a POST request to this endpoint will either
  * create a new `od_url_metrics` post, or it will update an existing post if one already exists for the provided slug.
  *
+ * @since 0.1.0
+ * @access private
  * @link https://google.aip.dev/136
  * @var string
  */
@@ -72,7 +76,7 @@
 			'required'          => true,
 			'pattern'           => '^[0-9a-f]+\z',
 			'validate_callback' => static function ( string $hmac, WP_REST_Request $request ) {
-				if ( ! od_verify_url_metrics_storage_hmac( $hmac, $request['slug'], $request['current_etag'], $request['url'], $request['cache_purge_post_id'] ?? null ) ) {
+				if ( '' === $hmac || ! od_verify_url_metrics_storage_hmac( $hmac, $request['slug'], $request['current_etag'], $request['url'], $request['cache_purge_post_id'] ?? null ) ) {
 					return new WP_Error( 'invalid_hmac', __( 'URL Metrics HMAC verification failure.', 'optimization-detective' ) );
 				}
 				return true;
@@ -214,6 +218,27 @@
 		);
 	}
 
+	/*
+	 * The limit for data sent via navigator.sendBeacon() is 64 KiB. This limit is checked in detect.js so that the
+	 * request will not even be attempted if the payload is too large. This server-side restriction is added as a
+	 * safeguard against clients sending possibly malicious payloads much larger than 64 KiB which should never be
+	 * getting sent.
+	 */
+	$max_size       = 64 * 1024;
+	$content_length = strlen( (string) wp_json_encode( $url_metric ) );
+	if ( $content_length > $max_size ) {
+		return new WP_Error(
+			'rest_content_too_large',
+			sprintf(
+				/* translators: 1: the size of the payload, 2: the maximum allowed payload size */
+				__( 'JSON payload size is %1$s bytes which is larger than the maximum allowed size of %2$s bytes.', 'optimization-detective' ),
+				number_format_i18n( $content_length ),
+				number_format_i18n( $max_size )
+			),
+			array( 'status' => 413 )
+		);
+	}
+
 	// TODO: This should be changed from store_url_metric($slug, $url_metric) instead be update_post( $slug, $group_collection ). As it stands, store_url_metric() is duplicating logic here.
 	$result = OD_URL_Metrics_Post_Type::store_url_metric(
 		$request->get_param( 'slug' ),
Index: types.ts
===================================================================
--- types.ts	(revision 3238281)
+++ types.ts	(working copy)
@@ -34,6 +34,7 @@
 
 export interface URLMetricGroupStatus {
 	minimumViewportWidth: number;
+	maximumViewportWidth: number | null;
 	complete: boolean;
 }
 

Index: includes/server-timing/class-perflab-server-timing.php
===================================================================
--- includes/server-timing/class-perflab-server-timing.php	(revision 3238281)
+++ includes/server-timing/class-perflab-server-timing.php	(working copy)
@@ -209,7 +209,7 @@
 	 */
 	public function use_output_buffer(): bool {
 		$options = (array) get_option( PERFLAB_SERVER_TIMING_SETTING, array() );
-		$enabled = ! empty( $options['output_buffering'] );
+		$enabled = isset( $options['output_buffering'] ) && (bool) $options['output_buffering'];
 
 		/**
 		 * Filters whether an output buffer should be used to be able to gather additional Server-Timing metrics.
@@ -220,7 +220,7 @@
 		 *
 		 * @since 1.8.0
 		 *
-		 * @param bool $use_output_buffer Whether to use an output buffer.
+		 * @param bool $enabled Whether to use an output buffer.
 		 */
 		return (bool) apply_filters( 'perflab_server_timing_use_output_buffer', $enabled );
 	}
Index: includes/site-health/audit-autoloaded-options/hooks.php
===================================================================
--- includes/site-health/audit-autoloaded-options/hooks.php	(revision 3238281)
+++ includes/site-health/audit-autoloaded-options/hooks.php	(working copy)
@@ -63,7 +63,7 @@
 		wp_die( esc_html__( 'Permission denied.', 'performance-lab' ) );
 	}
 
-	if ( empty( $option_name ) ) {
+	if ( '' === $option_name ) {
 		wp_die( esc_html__( 'Invalid option name.', 'performance-lab' ) );
 	}
 
Index: includes/site-health/audit-enqueued-assets/hooks.php
===================================================================
--- includes/site-health/audit-enqueued-assets/hooks.php	(revision 3238281)
+++ includes/site-health/audit-enqueued-assets/hooks.php	(working copy)
@@ -35,7 +35,10 @@
 
 			// Add any extra data (inlined) that was passed with the script.
 			$inline_size = 0;
-			if ( ! empty( $script->extra ) && ! empty( $script->extra['after'] ) ) {
+			if (
+				isset( $script->extra['after'] ) &&
+				is_array( $script->extra['after'] )
+			) {
 				foreach ( $script->extra['after'] as $extra ) {
 					$inline_size += ( is_string( $extra ) ) ? mb_strlen( $extra, '8bit' ) : 0;
 				}
@@ -78,7 +81,11 @@
 			}
 
 			// Check if we already have the style's path ( part of a refactor for block styles from 5.9 ).
-			if ( ! empty( $style->extra ) && ! empty( $style->extra['path'] ) ) {
+			if (
+				isset( $style->extra['path'] ) &&
+				is_string( $style->extra['path'] ) &&
+				'' !== $style->extra['path']
+			) {
 				$path = $style->extra['path'];
 			} else { // Fallback to getting the path from the style's src.
 				$path = perflab_aea_get_path_from_resource_url( $style->src );
@@ -89,7 +96,10 @@
 
 			// Add any extra data (inlined) that was passed with the style.
 			$inline_size = 0;
-			if ( ! empty( $style->extra ) && ! empty( $style->extra['after'] ) ) {
+			if (
+				isset( $style->extra['after'] ) &&
+				is_array( $style->extra['after'] )
+			) {
 				foreach ( $style->extra['after'] as $extra ) {
 					$inline_size += ( is_string( $extra ) ) ? mb_strlen( $extra, '8bit' ) : 0;
 				}
Index: load.php
===================================================================
--- load.php	(revision 3238281)
+++ load.php	(working copy)
@@ -5,7 +5,7 @@
  * Description: Performance plugin from the WordPress Performance Team, which is a collection of standalone performance features.
  * Requires at least: 6.6
  * Requires PHP: 7.2
- * Version: 3.8.0
+ * Version: 3.9.0
  * Author: WordPress Performance Team
  * Author URI: https://make.wordpress.org/performance/
  * License: GPLv2 or later
@@ -21,7 +21,7 @@
 }
 // @codeCoverageIgnoreEnd
 
-define( 'PERFLAB_VERSION', '3.8.0' );
+define( 'PERFLAB_VERSION', '3.9.0' );
 define( 'PERFLAB_MAIN_FILE', __FILE__ );
 define( 'PERFLAB_PLUGIN_DIR_PATH', plugin_dir_path( PERFLAB_MAIN_FILE ) );
 define( 'PERFLAB_SCREEN', 'performance-lab' );
@@ -104,11 +104,11 @@
 		),
 		'embed-optimizer'         => array(
 			'constant'     => 'EMBED_OPTIMIZER_VERSION',
-			'experimental' => true,
+			'experimental' => false,
 		),
 		'image-prioritizer'       => array(
 			'constant'     => 'IMAGE_PRIORITIZER_VERSION',
-			'experimental' => true,
+			'experimental' => false,
 		),
 		'performant-translations' => array(
 			'constant' => 'PERFORMANT_TRANSLATIONS_VERSION',
Index: readme.txt
===================================================================
--- readme.txt	(revision 3238281)
+++ readme.txt	(working copy)
@@ -2,7 +2,7 @@
 
 Contributors: wordpressdotorg
 Tested up to: 6.7
-Stable tag:   3.8.0
+Stable tag:   3.9.0
 License:      GPLv2 or later
 License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 Tags:         performance, site health, measurement, optimization, diagnostics
@@ -71,6 +71,12 @@
 
 == Changelog ==
 
+= 3.9.0 =
+
+**Enhancements**
+
+* Remove experimental flags from Embed Optimizer and Image Prioritizer. ([1846](https://github.com/WordPress/performance/pull/1846))
+
 = 3.8.0 =
 
 **Enhancements**

Index: helper.php
===================================================================
--- helper.php	(revision 3238281)
+++ helper.php	(working copy)
@@ -6,9 +6,11 @@
  * @package web-worker-offloading
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Gets configuration for Web Worker Offloading.
@@ -23,7 +25,7 @@
 	$config = array(
 		// The source code in the build directory is compiled from <https://github.com/BuilderIO/partytown/tree/main/src/lib>.
 		// See webpack config in the WordPress/performance repo: <https://github.com/WordPress/performance/blob/282a068f3eb2575d37aeb9034e894e7140fcddca/webpack.config.js#L84-L130>.
-		'lib' => wp_parse_url( plugin_dir_url( __FILE__ ), PHP_URL_PATH ) . 'build/',
+		'lib' => wp_parse_url( plugins_url( 'build/', __FILE__ ), PHP_URL_PATH ),
 	);
 
 	if ( WP_DEBUG && SCRIPT_DEBUG ) {
Index: hooks.php
===================================================================
--- hooks.php	(revision 3238281)
+++ hooks.php	(working copy)
@@ -6,9 +6,11 @@
  * @package web-worker-offloading
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Registers defaults scripts for Web Worker Offloading.
Index: load.php
===================================================================
--- load.php	(revision 3238281)
+++ load.php	(working copy)
@@ -15,10 +15,11 @@
  * @package web-worker-offloading
  */
 
-// Exit if accessed directly.
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
-	exit;
+	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 // Define the constant.
 if ( defined( 'WEB_WORKER_OFFLOADING_VERSION' ) ) {
Index: third-party/google-site-kit.php
===================================================================
--- third-party/google-site-kit.php	(revision 3238281)
+++ third-party/google-site-kit.php	(working copy)
@@ -6,9 +6,11 @@
  * @package web-worker-offloading
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Configures WWO for Site Kit and Google Analytics.
Index: third-party/seo-by-rank-math.php
===================================================================
--- third-party/seo-by-rank-math.php	(revision 3238281)
+++ third-party/seo-by-rank-math.php	(working copy)
@@ -6,9 +6,11 @@
  * @package web-worker-offloading
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Configures WWO for Rank Math SEO and Google Analytics.
Index: third-party/woocommerce.php
===================================================================
--- third-party/woocommerce.php	(revision 3238281)
+++ third-party/woocommerce.php	(working copy)
@@ -6,9 +6,11 @@
  * @package web-worker-offloading
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Configures WWO for WooCommerce and Google Analytics.
Index: third-party.php
===================================================================
--- third-party.php	(revision 3238281)
+++ third-party.php	(working copy)
@@ -6,9 +6,11 @@
  * @package web-worker-offloading
  */
 
+// @codeCoverageIgnoreStart
 if ( ! defined( 'ABSPATH' ) ) {
 	exit; // Exit if accessed directly.
 }
+// @codeCoverageIgnoreEnd
 
 /**
  * Adds scripts to be offloaded to a worker.

Index: deprecated.php
===================================================================
--- deprecated.php	(revision 3238281)
+++ deprecated.php	(working copy)
@@ -30,12 +30,17 @@
 	$metadata = wp_get_attachment_metadata( $attachment_id );
 
 	// Return full image size sources.
-	if ( 'full' === $size && ! empty( $metadata['sources'] ) ) {
+	if (
+		'full' === $size &&
+		isset( $metadata['sources'] ) &&
+		is_array( $metadata['sources'] ) &&
+		count( $metadata['sources'] ) > 0
+	) {
 		return $metadata['sources'];
 	}
 
 	// Return the resized image sources.
-	if ( ! empty( $metadata['sizes'][ $size ]['sources'] ) ) {
+	if ( isset( $metadata['sizes'][ $size ]['sources'] ) && is_array( $metadata['sizes'][ $size ]['sources'] ) ) {
 		return $metadata['sizes'][ $size ]['sources'];
 	}
 
Index: helper.php
===================================================================
--- helper.php	(revision 3238281)
+++ helper.php	(working copy)
@@ -66,7 +66,7 @@
 	// Ensure that all mime types have correct transforms. If a mime type has invalid transforms array,
 	// then fallback to the original mime type to make sure that the correct subsizes are created.
 	foreach ( $transforms as $mime_type => $transform_types ) {
-		if ( ! is_array( $transform_types ) || empty( $transform_types ) ) {
+		if ( ! is_array( $transform_types ) || 0 === count( $transform_types ) ) {
 			$transforms[ $mime_type ] = array( $mime_type );
 		}
 	}
@@ -163,7 +163,7 @@
 
 	$image_meta = wp_get_attachment_metadata( $attachment_id );
 	// If stored EXIF data exists, rotate the source image before creating sub-sizes.
-	if ( ! empty( $image_meta['image_meta'] ) ) {
+	if ( isset( $image_meta['image_meta'] ) && is_array( $image_meta['image_meta'] ) && count( $image_meta['image_meta'] ) > 0 ) {
 		$editor->maybe_exif_rotate();
 	}
 
@@ -185,7 +185,7 @@
 		return $image;
 	}
 
-	if ( empty( $image['file'] ) ) {
+	if ( ! isset( $image['file'] ) || ! is_string( $image['file'] ) || '' === $image['file'] ) {
 		return new WP_Error( 'image_file_not_present', __( 'The file key is not present on the image data', 'webp-uploads' ) );
 	}
 
Index: hooks.php
===================================================================
--- hooks.php	(revision 3238281)
+++ hooks.php	(working copy)
@@ -77,7 +77,7 @@
 		$metadata['sources'] = array();
 	}
 
-	if ( empty( $metadata['sources'][ $mime_type ] ) ) {
+	if ( ! isset( $metadata['sources'][ $mime_type ]['file'] ) ) {
 		$metadata['sources'][ $mime_type ] = array(
 			'file'     => wp_basename( $file ),
 			'filesize' => wp_filesize( $file ),
@@ -99,12 +99,12 @@
 	// Create the sources for the full sized image.
 	foreach ( $valid_mime_transforms[ $mime_type ] as $targeted_mime ) {
 		// If this property exists no need to create the image again.
-		if ( ! empty( $metadata['sources'][ $targeted_mime ] ) ) {
+		if ( isset( $metadata['sources'][ $targeted_mime ]['file'] ) ) {
 			continue;
 		}
 
 		// The targeted mime is not allowed in the current installation.
-		if ( empty( $allowed_mimes[ $targeted_mime ] ) ) {
+		if ( ! isset( $allowed_mimes[ $targeted_mime ] ) ) {
 			continue;
 		}
 
@@ -150,7 +150,7 @@
 			$original_image = wp_get_original_image_path( $attachment_id );
 
 			// If WordPress already modified the original itself, keep the original and discard WordPress's generated version.
-			if ( ! empty( $metadata['original_image'] ) ) {
+			if ( isset( $metadata['original_image'] ) && is_string( $metadata['original_image'] ) && '' !== $metadata['original_image'] ) {
 				$uploadpath    = wp_get_upload_dir();
 				$attached_file = get_attached_file( $attachment_id );
 				if ( false !== $attached_file ) {
@@ -171,7 +171,11 @@
 	}
 
 	// Make sure we have some sizes to work with, otherwise avoid any work.
-	if ( empty( $metadata['sizes'] ) || ! is_array( $metadata['sizes'] ) ) {
+	if (
+		! isset( $metadata['sizes'] ) ||
+		! is_array( $metadata['sizes'] ) ||
+		0 === count( $metadata['sizes'] )
+	) {
 		return $metadata;
 	}
 
@@ -179,7 +183,11 @@
 
 	foreach ( $metadata['sizes'] as $size_name => $properties ) {
 		// Do nothing if this image size is not an array or is not allowed to have additional mime types.
-		if ( ! is_array( $properties ) || empty( $sizes_with_mime_type_support[ $size_name ] ) ) {
+		if (
+			! is_array( $properties ) ||
+			! isset( $sizes_with_mime_type_support[ $size_name ] ) ||
+			false === $sizes_with_mime_type_support[ $size_name ]
+		) {
 			continue;
 		}
 
@@ -192,16 +200,16 @@
 		}
 
 		// The mime type can't be determined.
-		if ( empty( $current_mime ) ) {
+		if ( ! is_string( $current_mime ) || '' === $current_mime ) {
 			continue;
 		}
 
 		// Ensure a `sources` property exists on the existing size.
-		if ( empty( $properties['sources'] ) || ! is_array( $properties['sources'] ) ) {
+		if ( ! isset( $properties['sources'] ) || ! is_array( $properties['sources'] ) ) {
 			$properties['sources'] = array();
 		}
 
-		if ( empty( $properties['sources'][ $current_mime ] ) && isset( $properties['file'] ) ) {
+		if ( ! isset( $properties['sources'][ $current_mime ]['file'] ) && isset( $properties['file'] ) ) {
 			$properties['sources'][ $current_mime ] = array(
 				'file'     => $properties['file'],
 				'filesize' => 0,
@@ -217,7 +225,7 @@
 
 		foreach ( $valid_mime_transforms[ $mime_type ] as $mime ) {
 			// If this property exists no need to create the image again.
-			if ( ! empty( $properties['sources'][ $mime ] ) ) {
+			if ( isset( $properties['sources'][ $mime ]['file'] ) ) {
 				continue;
 			}
 
@@ -275,7 +283,7 @@
 	}
 
 	// Only setup the trace array if we no longer have more sizes.
-	if ( ! empty( $missing_sizes ) ) {
+	if ( count( $missing_sizes ) > 0 ) {
 		return $missing_sizes;
 	}
 
@@ -362,7 +370,7 @@
 function webp_uploads_remove_sources_files( int $attachment_id ): void {
 	$file = get_attached_file( $attachment_id );
 
-	if ( empty( $file ) ) {
+	if ( false === (bool) $file ) {
 		return;
 	}
 
@@ -371,7 +379,11 @@
 	$sizes = ! isset( $metadata['sizes'] ) || ! is_array( $metadata['sizes'] ) ? array() : $metadata['sizes'];
 
 	$upload_path = wp_get_upload_dir();
-	if ( empty( $upload_path['basedir'] ) ) {
+	if (
+		! isset( $upload_path['basedir'] ) ||
+		! is_string( $upload_path['basedir'] ) ||
+		'' === $upload_path['basedir']
+	) {
 		return;
 	}
 
@@ -383,7 +395,7 @@
 			continue;
 		}
 
-		$original_size_mime = empty( $size['mime-type'] ) ? '' : $size['mime-type'];
+		$original_size_mime = isset( $size['mime-type'] ) && is_string( $size['mime-type'] ) ? $size['mime-type'] : '';
 
 		foreach ( $size['sources'] as $mime => $properties ) {
 			/**
@@ -397,7 +409,11 @@
 				continue;
 			}
 
-			if ( ! is_array( $properties ) || empty( $properties['file'] ) ) {
+			if (
+				! isset( $properties['file'] ) ||
+				! is_string( $properties['file'] ) ||
+				'' === $properties['file']
+			) {
 				continue;
 			}
 
@@ -429,7 +445,11 @@
 			continue;
 		}
 
-		if ( ! is_array( $properties ) || empty( $properties['file'] ) ) {
+		if (
+			! isset( $properties['file'] ) ||
+			! is_string( $properties['file'] ) ||
+			'' === $properties['file']
+		) {
 			continue;
 		}
 
@@ -453,7 +473,7 @@
 			continue;
 		}
 
-		$original_backup_size_mime = empty( $backup_size['mime-type'] ) ? '' : $backup_size['mime-type'];
+		$original_backup_size_mime = isset( $backup_size['mime-type'] ) && is_string( $backup_size['mime-type'] ) ? $backup_size['mime-type'] : '';
 
 		foreach ( $backup_size['sources'] as $backup_mime => $backup_properties ) {
 			/**
@@ -467,12 +487,16 @@
 				continue;
 			}
 
-			if ( ! is_array( $backup_properties ) || empty( $backup_properties['file'] ) ) {
+			if (
+				! isset( $backup_properties['file'] ) ||
+				! is_string( $backup_properties['file'] ) ||
+				'' === $backup_properties['file']
+			) {
 				continue;
 			}
 
 			$backup_intermediate_file = str_replace( $basename, $backup_properties['file'], $file );
-			if ( empty( $backup_intermediate_file ) ) {
+			if ( '' === $backup_intermediate_file ) {
 				continue;
 			}
 
@@ -492,12 +516,16 @@
 	foreach ( $backup_sources as $backup_mimes ) {
 
 		foreach ( $backup_mimes as $backup_mime_properties ) {
-			if ( ! is_array( $backup_mime_properties ) || empty( $backup_mime_properties['file'] ) ) {
+			if (
+				! isset( $backup_mime_properties['file'] ) ||
+				! is_string( $backup_mime_properties['file'] ) ||
+				'' === $backup_mime_properties['file']
+			) {
 				continue;
 			}
 
 			$full_size = str_replace( $basename, $backup_mime_properties['file'], $file );
-			if ( empty( $full_size ) ) {
+			if ( '' === $full_size ) {
 				continue;
 			}
 
@@ -547,7 +575,7 @@
 	$image    = $original_image;
 	$metadata = wp_get_attachment_metadata( $attachment_id );
 
-	if ( empty( $metadata['file'] ) ) {
+	if ( ! isset( $metadata['file'] ) || ! is_string( $metadata['file'] ) || '' === $metadata['file'] ) {
 		return $image;
 	}
 
@@ -596,7 +624,7 @@
 		// Replace sub sizes for the image if present.
 		foreach ( $metadata['sizes'] as $size => $size_data ) {
 
-			if ( empty( $size_data['file'] ) ) {
+			if ( ! isset( $size_data['file'] ) || ! is_string( $size_data['file'] ) || '' === $size_data['file'] ) {
 				continue;
 			}
 
@@ -671,7 +699,7 @@
 	);
 
 	foreach ( $additional_sizes as $size => $size_details ) {
-		$allowed_sizes[ $size ] = ! empty( $size_details['provide_additional_mime_types'] );
+		$allowed_sizes[ $size ] = isset( $size_details['provide_additional_mime_types'] ) && true === (bool) $size_details['provide_additional_mime_types'];
 	}
 
 	/**
Index: image-edit.php
===================================================================
--- image-edit.php	(revision 3238281)
+++ image-edit.php	(working copy)
@@ -86,7 +86,11 @@
 		}
 
 		foreach ( $metadata['sizes'] as $size_name => $size_details ) {
-			if ( empty( $subsized_images[ $targeted_mime ][ $size_name ]['file'] ) ) {
+			if (
+				! isset( $subsized_images[ $targeted_mime ][ $size_name ]['file'] ) ||
+				! is_string( $subsized_images[ $targeted_mime ][ $size_name ]['file'] ) ||
+				'' === $subsized_images[ $targeted_mime ][ $size_name ]['file']
+			) {
 				continue;
 			}
 
@@ -124,7 +128,7 @@
 	}
 
 	$transforms = webp_uploads_get_upload_image_mime_transforms();
-	if ( empty( $transforms[ $mime_type ] ) ) {
+	if ( ! isset( $transforms[ $mime_type ] ) || ! is_array( $transforms[ $mime_type ] ) || 0 === count( $transforms[ $mime_type ] ) ) {
 		return null;
 	}
 
@@ -144,7 +148,7 @@
 			}
 			$callback_executed = true;
 			// No sizes to be created.
-			if ( empty( $metadata['sizes'] ) ) {
+			if ( ! isset( $metadata['sizes'] ) || ! is_array( $metadata['sizes'] ) || 0 === count( $metadata['sizes'] ) ) {
 				return $metadata;
 			}
 
@@ -163,7 +167,7 @@
 					}
 
 					if (
-						isset( $metadata['sizes'][ $size_name ] ) && ! empty( $metadata['sizes'][ $size_name ] ) &&
+						isset( $metadata['sizes'][ $size_name ]['file'] ) &&
 						$metadata['sizes'][ $size_name ]['file'] !== $old_metadata['sizes'][ $size_name ]['file']
 					) {
 						$resize_sizes[ $size_name ] = $metadata['sizes'][ $size_name ];
@@ -405,7 +409,7 @@
  * @param array<string, array{ file: string, filesize: int }> $sources       An array with the full sources to be stored on the next available key.
  */
 function webp_uploads_backup_full_image_sources( int $attachment_id, array $sources ): void {
-	if ( empty( $sources ) ) {
+	if ( 0 === count( $sources ) ) {
 		return;
 	}
 
@@ -435,7 +439,7 @@
 	$backup_sizes = get_post_meta( $attachment_id, '_wp_attachment_backup_sizes', true );
 	$backup_sizes = is_array( $backup_sizes ) ? $backup_sizes : array();
 
-	if ( empty( $backup_sizes ) ) {
+	if ( 0 === count( $backup_sizes ) ) {
 		return null;
 	}
 
Index: rest-api.php
===================================================================
--- rest-api.php	(revision 3238281)
+++ rest-api.php	(working copy)
@@ -30,7 +30,7 @@
 
 	foreach ( $data['media_details']['sizes'] as $size => &$details ) {
 
-		if ( empty( $details['sources'] ) || ! is_array( $details['sources'] ) ) {
+		if ( ! isset( $details['sources'] ) || ! is_array( $details['sources'] ) ) {
 			continue;
 		}
 
@@ -41,7 +41,13 @@
 	}
 
 	$full_src = wp_get_attachment_image_src( $post->ID, 'full' );
-	if ( ! empty( $full_src ) && ! empty( $data['media_details']['sources'] ) && ! empty( $data['media_details']['sizes']['full'] ) ) {
+	if (
+		isset( $full_src[0] ) &&
+		isset( $data['media_details']['sources'] ) &&
+		is_array( $data['media_details']['sources'] ) &&
+		isset( $data['media_details']['sizes']['full'] ) &&
+		is_array( $data['media_details']['sizes']['full'] )
+	) {
 		$full_url_basename = wp_basename( $full_src[0] );
 		foreach ( $data['media_details']['sources'] as $mime => &$mime_details ) {
 			$mime_details['source_url'] = str_replace( $full_url_basename, $mime_details['file'], $full_src[0] );