HTML was designed as an SGML application for networked hypertext. As an alternative to what the original post posits, we could leave it for that purpose, and design another (now XML) application for user interfaces: windows, buttons, scroll bars (if desired), text controls (no I'm not talking about textarea for CGI), the kind of controls that Windows or X11+MOTIF provide, expressed as tags in a UIML (User Interface M…
It never works. Same reason as to why adding non-JS languages never works: because making a GUI toolkit or implementing a language is a huge amount of work, other browser makers refuse to get on board because it'd mean they're playing catchup. Then web devs refuse to use it, because not every browser supports it. The only acceptable way forward is to gradually glue lots of small things onto HTML, and see which ones get implemented by the others. Because this is such an incremental and random process you end up with a pretty inconsistent platform that lacks a lot of stuff you'd intuitively expect.
[1] https://learn.microsoft.com/en-us/dotnet/desktop/wpf/app-dev...