The LLMs seem to struggle at anything that isn't relatively well anchored in whatever space. HTML documents have a lot of foundation to them in the training data, so they seem to perform well by comparison to other things.

I just spent a few hours trying to get GPT5.4 to write strict, git compatible patches and concluded this is a huge waste of time. It's a lot easier and more stable to do simple find/replace or overwrite the whole file each time. Same story in places like Unity or Blender. The ability to coordinate things in 3d is really bad still. You can get clean output using parametric scenes, but that's about it.