There are numerous IDEs accessible through the browser ranging from things like JSFiddle to VSCode and beyond.
I'm looking for something like an html/css/js app that acts like an IDE. Clearly, it won't be able to run any languages other than javascript or something ported to WebAssembly.
Edit
Based on @depperm's comment, I'm adding this: I think what I'm describing is a client-side app. I made this trivial example based on this codepen.