why not just add new xml/cool-web-application content type to all browsers that lives in a parallel universes next to text/html and allows building rich UIs in

why not just add new xml/cool-web-application content type to all browsers that lives in a parallel universes next to text/html and allows building rich UIs in wasm via nice modern api like for example flutter. Then HTML/CSS/JS could be left alone for documents and hypertext based pages like wikipedia, blogs etc and stuff like googles docs or figma can be built in a new format. And as both formats are supported by all browsers a document could just embed and xml/cool-web-application resource in an iframe and the application could just have a component to render an linked document.