QUAIL is a PHP library and jQuery plugin that lets you easily check HTML for adherence to accessibility standards. It comes with over 200 tests which implement Open Accessibility Tests and comes with WCAG 1.0, WCAG 2.0, and Section 508 guidelines.
Developers can build their own guidelines, or easily build a custom guideline that integrates with a database back-end or CMS. While the project supports checking entire HTML pages, integration with a CMS to check partial HTML content is probably the most popular use case.