What makes you certain any theoretical GUI markup standard would improve on the industry standard of Electron and Chromium wrapped web components? Just need to look at the utility and performance of vscode to see what is currently possible.
iOS doesn't even have a stable answer to React, yet. The problem isn't JS/CSS.
The Oracle Forms client seemed to somehow pull it off. (It used a proprietary markup, not XML.) There are deficiencies with the Oracle Forms model, but it proved a browser-like GUI client approach can work. Companies loved Oracle Forms before Java ruined it. They weren't pretty, but cheap and practical to create and maintain. The Oracle Forms devs were 3x more productive than our MVC team and it took about 1/5 the code to do the same thing. Productivity died in CRUDville; we de-evolved. (Same for desktop VB & Delphi like tools, although they required per-PC reinstalls for app upgrades, unlike Oracle Forms.)