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…

Nikoya

Check the doctype validity using W3c validator (html , xhtml , … ) when creating or updating page / post / custom post type and show the result in backend 
It show the result in back-end in sortable column, a link is add to the w3c for all file to help you to correct html error.
Very easy to see if a post / page is valid or not.
This plugin can check the W3C validity of your page / post or custom post type even if the site is not accessible from the Internet (if you work on local system for example)

(x)html easy validator

Check the doctype validity using W3c validator (html , xhtml , ... ) when creating or updating  page / post / custom post type and show the result in  …

phpHtmlLib vs (x)html easy validator