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.

WP-Validate

WP-Validate collects all the pages on your site and runs them through the W3C's HTML Validator.

Allan Collins

Is your website up to W3C HTML coding standards? This plugin will allow you to test each page or post on your site against the W3C Validator.
Want to contribute? Fork it: https://github.com/allan23/validated

Validated

This plugin will allow you to check your pages/posts HTML against the W3C Validator.

WP-Validate vs Validated