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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.

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.

FacadeX vs WP-Validate