@@ -163,6 +163,7 @@ public function data_provider_test_rest_api_availability(): array {
 	 * @covers ::od_compose_site_health_result
 	 * @covers ::od_get_rest_api_health_check_response
 	 * @covers ::od_is_rest_api_unavailable
+	 * @covers ::od_render_rest_api_health_check_admin_notice_in_plugin_row
 	 *
 	 * @dataProvider data_provider_test_rest_api_availability
 	 *
@@ -172,9 +173,23 @@ public function test_rest_api_availability( $mocked_response, string $expected_o
 		$this->filter_rest_api_response( $mocked_response );
 
 		$result = od_test_rest_api_availability();
-		if ( 'nginx_forbidden' === $this->dataName() ) {
-			$notice = get_echo( 'od_render_rest_api_health_check_admin_notice_in_plugin_row', array( 'optimization-detective/load.php' ) );
-			$this->assertStringContainsString( '</iframe>', $notice );
+		$notice = get_echo( 'od_render_rest_api_health_check_admin_notice_in_plugin_row', array( 'optimization-detective/load.php' ) );
+		if ( $expected_unavailable ) {
+			$this->assertStringContainsString( '<code>', $notice );
+			if ( is_array( $mocked_response ) ) {
+				if ( isset( $mocked_response['headers']['content-type'] ) && str_contains( join( '', (array) $mocked_response['headers']['content-type'] ), 'html' ) ) {
+					$this->assertStringContainsString( '</iframe>', $notice );
+					$this->assertStringNotContainsString( '</pre>', $notice );
+				} else {
+					$this->assertStringContainsString( '</pre>', $notice );
+					$this->assertStringNotContainsString( '</iframe>', $notice );
+				}
+			} else {
+				$this->assertStringNotContainsString( '</iframe>', $notice );
+				$this->assertStringNotContainsString( '</pre>', $notice );
+			}
+		} else {
+			$this->assertSame( '', $notice );
 		}
 		$this->assertArrayHasKey( 'label', $result );
 		$this->assertArrayHasKey( 'status', $result );
@@ -417,7 +432,7 @@ static function ( $pre, array $args, string $url ) use ( $mocked_response, $obse
 	}
 
 	/**
-	 * Build a mock response.
+	 * Build a mock JSON response.
 	 *
 	 * @param int                  $status_code HTTP status code.
 	 * @param string               $message     HTTP status message.
@@ -431,6 +446,9 @@ protected function build_mock_response( int $status_code, string $message, array
 				'message' => $message,
 			),
 			'body'     => wp_json_encode( $body ),
+			'headers'  => array(
+				'content-type' => 'application/json',
+			),
 		);
 	}
 }