I used to think KaTeX was far superior to MathJax but now I'm not so sure. I made https://mk12.github.io/web-math-demo/ to compare them and other things. They're definitely superior to browser MathML rendering today, which is nonexistent in Chrome (though see https://mathml.igalia.com/), quite bad in Safari, and OK in Firefox. It's true pre-rendering KaTeX produces a lot of markup, but it compresses very well so I don't think it's a big deal. They both have MathML for accessibility, but MathJax is more flexible in letting the user right-click and change the rendering engine, view raw TeX, etc. Rendering KaTeX on the client side is faster than MathJax, but in my experience KaTeX is slightly worse quality, e.g. https://github.com/KaTeX/KaTeX/issues/3400 has gone unfixed for a long time.
There's currently around 100 LaTeX functions listed as "Not supported" in the Katex docs at https://katex.org/docs/support_table.html I've been trying hard for a while with my site (https://cocalc.com) to use only katex, but that's definitely never going to happen. Users frequently hit missing functionality, e.g., they have lots of notebooks that use "\mbox", so it's critical to support full mathjax. I currently do this by attempting to use katex, then falling back to mathjax if it fails. That said, as you point out, mathjax has massively improved over the last few years with mathjax3! Thanks for pointing out the quality issues, which I hadn't thought about.
Sorry I posted the wrong issue, it’s actually a duplicate of https://github.com/KaTeX/KaTeX/issues/3168 which was filed last August. I guess it’s not that long, but it was annoying for my use of KaTeX on https://mitchellkember.com/notes4u where I see it frequently. For example it affects \left\lvert\vec{b}\right\rvert which is a pretty simple piece of TeX I’d expect to render fine.
