This is the live code editor used on WebGPUFundamentals, WebGLFundamentals, WebGL2Fundamentals, and the three.js manual.
It's based on the Monaco Editor which is the editor portion of Visual Studio Code
The goal was to be similar to JSFiddle or Codepen but client side only so there's several hacks. The biggest one is you call it with a url encoded in the query parameters. It then fetches that URL and parses it with brittle regular expressions. Because the input is under my control, it's only samples I've written or approved, I'm not too worried about using brittle regular expressions.
While parsing it needs to make all paths to external files to be fully qualified domain URLs. That is all links to images, videos, scripts, audio, CSS, workers, etc. It does some of this with user configured functions. The reason it needs to do this is because it runs the actual samples as blobs. There are no blob relative paths so all paths need to be fully qualified.
It has some support for handling workers, something even codepen and jsfiddle don't seem to easily support.
It also has support for providing editor line relative errors for JavaScript. In other words, JavaScript gets an error at line 475 in the actual blob that is running but in the editor in JavaScript it might be line 17. The JavaScript errors can be caught via a helper that is inserted into the blob which are then sent to the editor which can generate a relative line number and then move the cursor to the appropriate line.
The biggest drawback is the JavaScript debugger in the browser will lose all breakpoints every time the user's code is run since a new blob is generated so the debugger can't associate previous breakpoints.
In its current form it is probably not entirely stand alone and is pretty hacky. I just separated out as I finally got tired of manually propogating changes between repos.