ValidateHTML
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Free HTML5 Validator

Check your HTML5 code against the latest web standards. Catch deprecated elements, missing attributes, and specification violations instantly.

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What Does an HTML5 Validator Check?

HTML5 introduced significant changes to the web standard, including new semantic elements like <article>, <section>, <nav>, and <header>. At the same time, many older elements were deprecated.

Our HTML5 validator checks for deprecated elements like <center>, <font>, <marquee>, and <blink>. It also verifies that required attributes are present, like alt on images and proper DOCTYPE declarations.

Common HTML5 Issues

Missing DOCTYPE

HTML5 requires <!DOCTYPE html> at the top of every document.

Deprecated Elements

Tags like <center>, <font>, and <big> are no longer valid in HTML5.

Missing Alt Attributes

Images must have alt text for accessibility and HTML5 compliance.

Incorrect Nesting

Block elements inside inline elements violate HTML5 content models.

Missing Lang Attribute

The <html> element should have a lang attribute for accessibility.

Unclosed Tags

While HTML5 is more lenient, unclosed tags can cause unpredictable rendering.

Valid HTML5 That Looks Wrong

HTML5 is more permissive than its reputation. Each of these returns zero errors, because the specification allows it, and a validator that flagged them would be teaching you rules that do not exist:

  • Unquoted attribute values<p class=intro>
  • Omitted end tags<p>One<p>Two
  • Custom elements<my-widget>...</my-widget>
  • Popover and dialog<div popover id=menu>

Errors and Warnings Are Not the Same

An error here means the HTML breaks the WHATWG specification: <center> (removed from HTML), an <img> without alt, a <div> directly inside a <ul>, two elements sharing an id, a <div> inside a <p>. A warning means the HTML is valid but falls short of a best practice, such as an <iframe> with no accessible name.

The difference matters because most of the web fails the first bar. In our crawl of the Tranco top 5,000, 87.2% of the 2,656 sites we could read break the HTML specification on their home page. The full study lists the most common errors, and each one links to a guide to fixing it.

Frequently Asked Questions

What is the difference between HTML5 and HTML4 validation?
HTML5 introduced new semantic elements (article, section, nav, header, footer, main), new input types (email, date, range), and deprecated many presentational elements (center, font, big, strike). An HTML5 validator checks against these modern rules, while HTML4 validators use the older, stricter DTD-based specification. Most websites today should validate against HTML5.
Is HTML5 validation different from regular HTML validation?
HTML5 validation focuses specifically on the HTML5 specification. It catches deprecated elements that were valid in HTML4 but are no longer part of the standard, verifies new HTML5 attributes and elements, and checks for the simplified DOCTYPE declaration. Regular HTML validation may not flag these HTML5-specific issues.
What are semantic HTML5 elements and why do they matter?
Semantic elements like <header>, <nav>, <main>, <article>, <section>, and <footer> describe the meaning of their content rather than just its appearance. They improve accessibility by helping screen readers understand page structure, improve SEO by giving search engines clearer content signals, and make your code more readable and maintainable.
Why does the DOCTYPE declaration matter in HTML5?
The <!DOCTYPE html> declaration tells browsers to render the page in standards mode rather than quirks mode. Without it, browsers may use legacy rendering behavior that causes inconsistent layouts across different browsers. In HTML5, the DOCTYPE is simplified to just <!DOCTYPE html>. No DTD reference is needed.
How is this different from the W3C Nu HTML Checker?
The W3C Nu HTML Checker is the official validator backed by the W3C and is the most authoritative source for HTML5 conformance. It is highly accurate but has a dated interface and gives minimal context for fixing errors. Our HTML5 validator runs the same core rules in a faster, modern UI with clearer error messages and a quality score. Both tools agree on what counts as invalid HTML5; ours is built to make iteration easier.
Should I worry about HTML5 validation errors that do not affect rendering?
Yes, in most cases. Browsers are forgiving and will render technically invalid HTML5 without visible issues, but invalid markup can cause problems for accessibility tools, search engine crawlers, RSS readers, and email clients. Validation errors also accumulate technical debt: code that renders today may break with browser updates or refactoring later.
Is this HTML5 validator free to use?
Yes, our HTML5 validator is completely free with no usage limits. You can validate as many HTML5 documents as you need without registration. Just paste your code and get instant results.