Html5 video has so many problems, that its just plain boring (that includes slow webm encoding, no full-screen, browsers not being honest about their abilities to play certain format, ect), so what do you plan to achieve?
some older detection test:
(chromium certainly does not play h.264 without additional system decoders.., basically a detection library would have to know every single browser by its name, boring.)
Even if you solve all the mysteries and try to do a webm web video delivery, you will still end-up with some sort of (silly) library (or better: user deciding what would want - switches) that will have to decide what to deliver to each and every browser (flash, html5, quicktime maybe?) - and i'am not even counting different ipods here. So, say the delivery would want to be something else than your usual youtab clone, you are into a huge coding pain (And i'am not saying it is not solvable, i just didn't happen to see any decent solution till now).