I've been looking for something similar that either has a markup language associated with it or is easy to add one to. I wish to prototype a GUI markup standard

I've been looking for something similar that either has a markup language associated with it or is easy to add one to. I wish to prototype a GUI markup standard with it as a proof of concept so that we can have GUI-over-HTTP instead of live with the ever failing & bloated HTML/JS/DOM/CSS GUI emulators. So far, Terminal.Gui looks interesting, but I don't know if it's cross-platform yet.