The language in the browser for specifying what should show up and in what format is HTML and CSS. We can't remove them because we don't have anything to substitute; without them, there's just no displayable content.
Is your proposal that we replace those relatively heavyweight standards with something more primitive that we could then build the behavior on top of? I think there's meat on those bones. Quite frankly, the amount of work we do to push intent to fit the constraints of HTML and CSS in web apps is a little absurd relative to the frameworks and languages we have to do that in non-web widget toolkits. I'm not actually convinced that "Tk as an abstraction in the browser that we build HTML and CSS on top of" would be a bad thing (although we probably want to use something better than Tk, with more security guarantees).
... However, if we did that, we would really damage the accessibility story as it currently stands (since accessibility hinting is built on top of the HTML spec) and that's probably a bridge too far. We already have enough site developers who put zero thought into their accessibility; removing even the defaults HTML provides with its structure would be a bad call.