On the extreme end, a web app can do all of its own rendering in a canvas with WebGL/WebGPU. Some apps do exactly this: Figma, Google Maps, Google Docs. Just to name a few. (edit: Earlier I claimed PDFjs uses canvas, but it does not. I was confusing it with Google Docs [1].)

It's a thing you can do. But it is very bad for extensions and extension developers for the same reasons that Java applets, Flash, and Shockwave were bad for the web. These apps are difficult for end users to customize. It's a real bane to tinkerers. And it's a shame that "view source" has slowly grown completely useless over the decades.

[1]: https://workspaceupdates.googleblog.com/2021/05/Google-Docs-...