What determines the required compatibility in the WCAG 2.0 guidelines?
If everything works perfectly in FF/Chrome/Edge but not in IE11 or 10, or 7 or 5, is this a fail?
If it is a fail, is there a place that lists minimum compatibility? I assume that if it fails using Netscape, It'll still pass the audit. So, who decides the minimum compatibility requirement and where is that listed?
WCAG does not define a recommended or even a required set of browsers (the most common type of "user agents") for conformance. Browsers may get updated several times per year, whereas updating WCAG takes several years. Instead of specifying browsers, WCAG relies on the concept of "accessibility supported technologies" or, more precisely, "accessibility-supported ways of using technologies" (as described in the conformance section):
Only accessibility-supported ways of using technologies are relied upon to satisfy the success criteria. Any information or functionality that is provided in a way that is not accessibility supported is also available in a way that is accessibility supported. (See Understanding accessibility support.)
("Technologies" here refers to content formats, stylesheet mechanisms and scripting languages used on the web, e.g. HTML, CSS, JavaScript, SVG, PDF, etc.)
WCAG defines accessibility supported as follows:
supported by users' assistive technologies as well as the accessibility features in browsers and other user agents
This definition is followed by a set of criteria for "accessibility supported". Note that it does not only mention "browsers and other user agents" but also to assistive technologies. It would not be sufficient to only mention browsers, since users also rely on assistive technologies such as screenreaders, and some screenreader-browser combinations work better than others. See for example the section Screen Reader / Browser Combinations in WebAIM's seventh screenreader survey. JAWS is used more frequently with Internet Explorer than with Chrome, partly because it works better with IE. NVDA is used more frequently in combination with Firefox than with Chrome or Internet Explorer because its compatibility with Firefox is better.
WCAG does not define a list of browsers, nor how many generations of older browsers should be supported. The section Understanding Accessibility Support in Understanding WCAG 2.0 says (emphasis mine),
Many of the Success Criteria deal with providing accessibility through assistive technologies or special accessibility features in mainstream user agents (for example, a 'show captions' option in a media player).
WCAG does not define "mainstream".
Understanding WCAG 2.0 goes on to say (emphasis mine),
The Working Group, therefore, limited itself to defining what constituted support and defers the judgment of how much, how many, or which AT must support a technology to the community and to entities closer to each situation that set requirements for an organization, purchase, community, etc.
Delegating this type of decision is necessary, since accessibility support is not an exclusively technical matter: what browsers and assistive technologies are available differs from country to country (and hence from jurisdiction to jurisdiction) because some countries are quicker to adopt newer versions of browsers and AT because of economic and other reasons. For example, in countries where the government refunds Braille displays, AT that works well with Braille displays (instead of just TTS) is more common than in countries where no such government support exists. Hence, accessibility support for certain technologies and techniques will become available more quickly in countries where people can afford to upgrade to newer browsers and AT versions.
So whether a site passes an accessibility evaluation may depend on where that accessibility evaluation is done.
If everything works perfectly in FF/Chrome/Edge but not in IE11 or 10, or 7 or 5, is this a fail?
WCAG makes no mention of compatibility with specific browsers.
It'll still pass the audit. So, who decides the minimum compatibility requirement
WCAG is not a set of sharply designed rules. They are guidelines with some room for interpretation. So the answer to that is "Whomever is doing the audit".
