TL;DR about the specific problem I was having with HTML-CSS:
> both HTML-CSS and CommonHTML use web fonts. But CommonHTML doesn't try to
> detect their presence, while HTML-CSS does, and it is that detection that is
> probably going wrong for some reason (I did say it was fragile). HTML-CSS
> needed to wait for the fonts to arrive because (in the old days before
> browsers were more consistent with each other), MathJax had to measure the
> actual displayed subexpressions in order to determine their size, and that
> meant the font had to be the right one. CommonHTML doesn't have to do that
> because modern browsers are more consistent about their layout (thanks to
> better standards and better adherence to them).
