HTML does describe GUIs though. Browsers are engines that turn a text description of a GUI into a rendered GUI.

I get your point, it does feel like there's something different, but I'd suggest that there is no black and white line, instead there's a spectrum, with fully native local UI at one end and fully declarative remote GUI at the other end. HTML and the web exist somewhere towards the latter.

There have been many attempts at points all the way along that line though, and every point comes with trade-offs. Performance degrades, OS-native components are eschewed, programming gets higher level, etc.