WP-Validate
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WP-Validate collects all the pages on your site and runs them through the W3C's HTML Validator.
WP-Validate indexes your site, collecting all the blog posts, pages and other published content, and submits it to the W3C’s HTML Validator, which then returns the response.
WP-Validate then lists the valid pages, and the invalid pages, with a link to the specific validator URL.