Mike Walsh

phpHtmllib is a set of PHP classes and library functions to help facilitate building, debugging, 
and rendering of XML, HTML, XHTML, WAP/WML Documents, and SVG (Scalable Vector Graphics) images 
as well as complex html ‘widgets’. It provides a mechanism to output perfectly indented/readable 
tags, and a programmatic API to generating tags on the fly.
More information can be found on the <a href="http://michaelwalsh.org/wordpress/wordpress-plugins/phphtmllib/" rel="nofollow ugc">phpHtmlLib</a> 
plugin page.

phpHtmlLib

The phpHtmlLib library contains a set of PHP classes and library functions to help facilitate…

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.

phpHtmlLib vs WP-Validate