That is so informative.
Seriously though, wouldn't one expect the authors of this spec to be experts on web technologies who eat their own dog food (of web standards) instead of relying on a pale imitation of a dated typesetting technology like TeX?
You seem to be confusing "mature" with "dated."
- despite the window dressing of LaTeX, it is ultimately presentation-oriented instead of structure-oriented; many widely-used forms of formatting markup rely on precise character dimensions and positioning; changing page dimensions may sometimes require fixes throughout the whole document
- bad character set support: things like \mathcal{3} displays the wrong character WITH ZERO WARNING: see https://tex.stackexchange.com/q/84041/ and duplicates
- parsing is undecidable; the parser can be arbitrarily reconfigured at runtime, and can perform Turing-complete computations, which means processing arbitrary TeX code in any other way than to execute it as written is next to impossible; even a 100% correct syntax highlighter is unachievable
MathJax fixes or ameliorates some of these flaws, but in trying to be compatible with TeX, it cannot avoid all of them; and in the meantime, it imposes its own syntax quirks and burdens the client with running bloated JavaScript, incurs significant pop-in latency, and leaves indecipherable mess of TeX code to clients which do not run JS. TeX is just PDF that's easier to edit, and MathJax is the Cufon of math markup. (Remember Cufon? Be thankful if you don't. Good riddance!)
And yet Knuth seems to regard Unicode not as a basic building block of interoperability, but as useless bells-and-whistles, judging by his satirical "iTeX" presentation.
FWIW, it works perfectly fine for me on Linux with both Chrome and Firefox, so I'd say probably something is wonky in your environment, or a temporary issue on the website.
It’s really a truly terrible spec site.