QUAIL is a jQuery plugin for checking content against accessibility guidelines. It provides a flexible way to test for certain problems (say, images missing an alt text) and a collection of over 200 tests to get you started. Defining your own tests is easy, and you can pick-and-choose tests to focus on the needs of your own app.
QUAIL has just made a big move from a humble PHP library to a lightweight and powerful client-side accessibility checker. You can still find the 1.0 version of QUAIL, or check out the future of QUAIL. A full test suite is available for your perusal.
Documentation
You can read the latest documentation, hosted at readthedocs.org.
Demonstration
There are several accessibility problems in here, but QUAIL will find them.
The code
$('.demonstration').quail({
jsonPath : '/js/quail/src/resources',
guideline : [ 'imgHasAlt',
'documentAbbrIsUsed'
],
testFailed : function(event) {
...code to highlight problems...
}});