Feature Strings

Forgive me, I have not gone through WHATWG's HTML5 doc cover-to-cover yet.

What do people think about feature strings and the <switch> element in
SVG?  Personally I think it's a good way to allow implementors to
incrementally add support for a spec and a great way for developers to
figure out fall-back content.  User agent sniffing always seemed so
brittle to me.

In WHATWG's HTML5, is there a mechanism to determine if a UA supports
the <video> element yet?  Canvas is probably done with some JS and an
exception catching mechanism, right?

Thanks,
Jeff