edik

If FacadeX is active, the public WordPress sites are sent with the xhtml mime type. The internal sites like the administration panel and the login screen, which have many xhtml syntax errors, are still sent with the standard mime type.
You must ensure that your public sites have valid XHTML, otherwise you will get parsing errors in your browser.

FacadeX

Sends public sites with the XHTML mime type. The internal sites, which have many XHTML errors, are still sent with the HTML mime type.

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.

FacadeX vs Validated