Compared to the amazing complexity of OpenType layout for complex scripts, mathematical typesetting is pretty simple. It's just not been a priority for the browser vendors, in the same way that we now have amazing technologies like WebRTC and WebGL but managing bookmarks still sucks.
It might "simply be" the case (well, in fact it is) that TeX is simply really old and doesn't lend itself well to concepts it predates such as dynamic reflow, etc...
It might "simply be" the case (well, in fact it is) that TeX is simply really old and doesn't lend itself well to concepts it predates such as dynamic reflow, etc...