[whatwg] HTML5 video element - default to fallback in cases where UA can't play format

[Kit Grose]

It seems counterintuitive to me that having produced fallback content  
already, I still need to use Javascript to test for compatibility  
(even if I *did* generate two formats, there's obviously no guarantee  
IE9 won't come out requiring WMV or a similar issue with a different  
UA).

Are there any other elements in HTML that _require_ Javascript to  
provide a good UX?

—Kit