Or, as it historically been the case, a browser vendor implements the feature (so it's "supported") but they do it wrong.

As much as people hate to hear it, the only real solution is some kind of browser-version tagging. If it wasn't for Microsoft's browsers supporting browser-specific conditional comments, we'd have a much harder time writing standards-compliant web pages that work the same for every browser.

Using hacks to work around bugs, is tolerable, perhaps. But for the common case where a feature is only partially implemented across a swath of browsers, I think feature inspection works great. It won't be a panacea, but it will be a step in the right direction.