HTML 5 has a long way to go, not all features are supported across all platforms and even the single-most simplest animation or interactivity will require a lot of lines of code to function properly, let alone the extra lines of code you would end up coding to make it work across all browsers.

Actually, the "single-most simplest animation or interactivity" (plus things far more advanced that that) can be achieved with like 1-2 lines of javascript, and sometimes not even one line, using declarative css transitions and such.