The World Wide Web Consortium (W3C) is an international community where Member organisations, a full-time staff, and the public work together to develop Web standards.
It's worth bearing in mind that many of the checks below are similar or identical to those used in Search Engine Optimisation (SEO). As a result, you may find this checklist much less of a burden than you think in terms of web content.
If your web/marketing team are practising SEO (as they should be!), then much will already be covered. So, if you are trying to sell the need for accessibility internally, remember it's good for traffic, it's good for usability, and it's the law!