We tested against 2.1 AA requirements, the current best practice for digital accessibility standards. The open source tools we used were Domain Accessibility Audit (to crawl the pages) and axe (to perform the actual accessibility scans).
These problems are happening because accessibility has been an afterthought, even in government. In order for accessibility success to scale in the enterprise we need to take a more holistic approach. For this we have developed our HEART framework.
Learn more by reading our NEXTGOV/FCW article.