User Agent Accessibility Guidelines (UAAG) 2.0 is a W3C working group note especially aimed towards user agent developers “to help them design user agents that make the Web more accessible to people with disabilities”. User agent includes software such as browsers, browser extensions, media players, and any application and/or system capable of rendering web content.
By complying with UAAG 2.0, user agents will be able to improve web accessibility by providing their own features wherever appropriate, and communicating properly with other technologies (including assistive technologies). A user agent compliant with UAAG can benefit everyone, not only users with disabilities!
Generally speaking, as a website developer or content author you should rarely need to consult this resource, unless you want to dig deeper into implementation details or understand how web accessibility works “under the hood”. However, if you are interested this resource could help you to understand the reason why accessibility-features are designed the way they are.