This plugin will allow you to check your pages/posts HTML against the W3C Validator.
This plugin will send the URL of your page or post to the W3C Validator to verify that you have valid HTML code.
No. If your page or post is invalid, a link to the W3C Validator is provided to show you what needs to be fixed.
Verify that your page or post URL can be accessed publicly. At this time, this plugin cannot test local/private development installations. If you still encounter errors, please let me know by submitting it to the support forum here: http://wordpress.org/support/plugin/validated