I do write the raw HTML actually
In either case, you can use TeX as your input, and if you do, you have to convert it, client-side or browser-side, into something usable by the browser; it's just that if the browser accepts MathML the rendering is faster and/or more convenient, plus you get other options.
I’m not claiming that a JavaScript parser and complete renderer for TeX is better than a JavaScript parser that renders via MathML. The second option may indeed be more efficient - but making the browser parse a sensible notation for maths instead of an XML crapfest would be better than either.