Can you list _worthwhile things had workable implementations before HTML 5_ and the stuff they implemented before HTML5?
<canvas>? <video>? local storage?
With Canvas, you might have a point, though it makes a poor justification for the rest of HTML 5. I've yet to need Canvas for anything, though. Video is silly to even ask about. Google Gears rather predated HTML 5 local storage.
Now, give me something I couldn't do before with nearly-universal plugins.
