WordPress plugin that validates your site's HTML and reports any errors. Works for local sites too!
Install and activate it as you would any other plugin. Then visit any webpage on your site (on the front end or the administration panels). You can scroll down to the footer and see whether the HTML is valid.
The results are cached per-URL using transients, and the default expiration is 2
minutes. You can change this by defining WP_HTML_VALIDATOR_CACHE_EXPIRATION in your
wp-config.php:
define( 'WP_HTML_VALIDATOR_CACHE_EXPIRATION', 15 * 60 ); // 15 minutes