[jed_watson]

I was just thinking a similar thing. If user agents have evolved to effectively become a way of saying "Our browser supports standards X" (where X is an awkward grouping based on which features vendors and browser versions originally supported the feature / standard), surely the ultimate solution is to provide a header enumerating support on a per-feature basis.

Obviously it would take a while for the standard to be adopted to the point where it was useful, but as a web developer I dream of a time when we receive a header that lists the supported features in the client (similar to the classnames modernizer adds to the document body, and probably eventually versioned) and no longer have to resort to either UA parsing OR javascript-based feature detection, both of which are hacks because this problem hasn't actually been solved properly at the standards level.