When I do an audit of a web-based system against WCAG, I subject each test unit to both automated & manual tests against nearly 250 Best Practices. Of those, 40 of them relate directly to forms. The #1 way which users interact with web-based systems is through the use of forms. Some sites, such as Facebook & Twitter are nothing more than forms and content generated by users interacting with those forms. Naturally it makes sense to ensure that the forms are both usable and accessible