"HTML 5" has become a very loosely defined set of techniques. It generally means stuff that recently became possible with modern browsers. Thus, you can't expect any of it to work on older browsers, like IE 6 or even FF 3.0.
Also be aware that "all platforms" really is a lot of platforms these days, even if you just count the modern ones. You might want to make sure it works on touch-devices, which means hovering can't be used for input.
There is also a wide range of performance on the web. Anything from cheap Android phones to high-end desktop PC:s.