Ah ok, I took you to be asking what's needed for HTML5 technologies to replace LaTeX/ConTeXt and similar as general tools for typesetting, which would include the ability to target both paper and screen output.

If it's for electronic documents purely, then I think we're pretty close, at least in terms of what the technology supports. Making it easy to do various kinds of typesetting tasks are perhaps another story; there is a lot of stuff that is technically possible with the tech modern browsers support but not easy to do, certainly not with a workflow as automated as LaTeX's.