I want to use \underbrace in my slides to add some notes to maths.
Version 0.8.0 of KaTeX added support for \overbrace and \underbrace (see https://github.com/Khan/KaTeX/releases/tag/v0.8.0) so it should be fine if you update KaTeX to the latest one.
KaTeX has been updated to 0.10.0, so we now support \underbrace as well as all of the functions listed here: https://khan.github.io/KaTeX/docs/supported.html
As part of this we’ve made it easier to update KaTeX versions in the future so we can avoid falling so far behind again 🚀
Zachary Sunberg commented
In latex \underset does not appear to work, even though the katex documentation says that it is supported https://khan.github.io/KaTeX/function-support.html