HTML_CodeSniffer – Check Any HTML With The Given Web Standard
HTML_CodeSniffer is a bookmarklet to check if a web page validates for the selected standard.
The bookmarklet is open source, has a slick interface and currently comes with a set of 3 standards that enforce the Web Content Accessibility Guidelines (WCAG) 2.0.
It displays the errors, warnings + notices and their details can still be viewed within the bookmarklet.
The standards and their rules are hosted in the GitHub repository, so, once a new standard or rule is added by the developer, the bookmarklet will auto-update.