Isn't it time for user agents to start reporting more fine grained which standards they support? The HTTP Accept header doesn't provide enough information to know whether a document will be understood at all, and can lead to quite a few hacks, specially on sites using cutting edge technology, such as SVG or AJAX.
For an example, take a look at the correspondence between the Firefox 188.8.131.52 Accept header (text/xml, application/xml, application/xhtml+xml, text/html;q=0.9, text/plain;q=0.8, image/png, */*;q=0.5) and the support level reported at Web Devout's Web browser standards support page or Wikipedia's Comparison of web browsers.
Say that I want to serve a page with some SVG, MathML, CSS2/3, and AJAX functionality. Each of these requires different hacks to ensure that non-compliant browsers don't barf on the contents. For SVG and MathML, I can use CSS to put the advanced contents above the replacement images, or use e.g. SVG to provide replacement text. Both methods increase the amount of contents sent to the user agent, and are not really accessible - Non-visual browsers get the same information twice.
For CSS, countless hacks have been devised to make sure sites display the same in different browsers. So the user agent always receives more information than it needs.
What if browsers could negotiate support with the server using e.g. namespace URIs, where these would reference either a standard, part of it, or some pre-defined support level? Poof, SVG 1.1 Tiny 95% supported, CSS 3 10% supported, DOM level 2 80% supported, etc..
Obviously, the Accept header would be much longer, but the contents received could be reduced significantly. Also, I believe it would be easier for developers to use only Accept header switching than learning all the hacks necessary for modern web development.
I don't really know if this is possible, but maybe this kind of Accept header could be separated into a special HTTP reply. This would contain the URIs of the potential contents, and the user agent would send a new HTTP GET request with the modified Accept header, reporting the support levels.