Tree-sitter does run on the web. I got it working for my editor, but it did involve several days' worth of effort and getting into the weeds with emscripten. Details here - https://gushogg-blake.com/p/emscripten-web-modules/.
I just discovered lezer and am tinkering with integrating it into my projects, but maybe tree-sitter would be a better fit?
(Speed could be a determining factor, but working with wasm can also make things slower, if you often have to get data in and out)
Having used Lezer and CodeMirror a bit on a different project, they both seem like they are probably high quality and well thought out projects from a speed and reliability perspective, but I found the architecture and docs to be confusing and unergonomic.
The main issue I have with the docs/architecture (for CM at least) is that they use a concept called "facets" without really explaining what it means, and to be honest it felt like a level of abstraction/indirection that my brain couldn't handle.
The other issue with the docs is that they don't seem contextualised enough, somehow. They would list the functions/methods of an object but not really explain how it fit into the system as a whole. Maybe this is to do with not understanding facets, or CM's extension architecture, enough, or something, but it was definitely a recurring theme of my experience. I also found the CM API slightly confusing in that (almost) everything you do is via a function that takes some state object as input, as opposed to via calling methods on that object, but that's more of a style issue once you realise that you have to `import {doSomething} from "codemirror"` and `doSomething(state)` as opposed to `state.doSomething()`.
And since I use codemirror anyway, it might make sense to also use lezer, unless there are downsides I am not aware of.
When you need to build a general tool that supports a ton of languages, that’s when Lezer doesn’t fit as well since there just aren’t a ton of existing grammars out there.