Presumably any app that wants this kind of rendering could make use of it, not just browsers--consider all the apps that now use Electron for this.
There are a lot of applications where you'd want to take advantage of a browser view or a renderer or a JS engine, but not the rest of the stack. There are native apps where you might want a well-sandboxed JS engine for extension support, or where you're writing all of your core logic in C/Rust but you want to use HTML/CSS for your interface.
Splitting up those components would be useful in a lot of situations.
DOM is highly intertwined with JavaScript, so that wouldn't be possible.
I would love to be able to contribute improvements to the desktop environments I use, but I don’t have the time to learn languages that aren’t applicable to my daily work.