The HTML5 Validator can be used to validate WAI-ARIA markup.
Author: Gez Lemon
I found out today from Henri Sivonen on the Free ARIA list that the HTML5 validator can be used to validate ARIA markup. This is incredibly useful, as up to now, the only way I knew of verifying ARIA was to test with assistive technology. Of course, it's always good to test with assistive technology, but it's more convenient to use a validator while developing.
Using the validator will obviously give errors for things that aren't in the HTML 5 specification, such as using an HTML 4.01 doctype. But they can be ignored, or you can use the direct input just to add the ARIA parts you want to validate. This is a great discovery that will make authoring ARIA a lot easier.