Me too! When @supports was proposed I made sure to propose a JS API for the same functionality. David Baron liked the idea and it's now (mostly) in the spec: http://dev.w3.org/csswg/css3-conditional/#window-api

Right after @supports landed I got a bug filed for supportsCSS() https://bugzilla.mozilla.org/show_bug.cgi?id=779917 and a few days later an initial patch landed. So I think we're all set here.

Also on browser support: Opera already has an implementation in progress of @supports; we haven't seen patches yet from WebKit but there is definitely interest.