It looks like the live demo is no longer up, but it did run latex in the browser and render the dvi output to html. The wasm for TeX is about 495kb / 88kb compressed, but the memory image for LaTeX was a bit larger.
I would find it hilarious if LaTeX turned into a leader in that space. I doubt it could hold on to that. There's a lot of things that something designed from the beginning for web-like uses could probably improve on that would be capable of overcoming LaTeX. But I could see a world where it carves out a niche and holds on to that niche for a long period of time.
The things you can't do are things like expose an accessibility tree (without a dummy DOM), interact with the system IME, and access system fonts.
Even input you might be able to hack around but accessibility is a big deal and the "hack" at that point is nearly to both lay it out in the browser and the supposed "fixed" layout system, and while that may work in some sense I again have lots of questions about whether that is really "fixed".
FWIW, I actually think it would be much more valuable just to fix the spec and make CSS layout fast-by-default.
Nobody else seemed convinced :(
This is pdfTeX, Version 3.14159265-2.6-1.40.21 (SwiftLaTeX PDFTeX 0.3.0) (preloaded format=swiftlatexpdftex) I can't find the format file `swiftlatexpdftex.fmt'!
Likewise for XeTeX
This is hillarious. Browsers lost the ability to print some 10 years ago. Today, printing a web page is an exercise in masochism.
I am very curious how the output will look like.