It's relatively easy to force your custom stylesheets to any website. Whether it will work at all is another question.
I believe what you suggest could work only with semantic HTML without any advanced styling - no floating elements, no flexbox/grid, no absolute/relative positioning etc. This then means return to static text based websites which is fine for many use cases, but not for all.
Ideally, rich web apps which actually need all the CSS/APIs could use them, while the text-based sites would keep custom styling/JS to minimum (which would allow reasonable user-defined styling). But it's impossible to force this dichotomy in practice.
I believe what you suggest could work only with semantic HTML without any advanced styling - no floating elements, no flexbox/grid, no absolute/relative positioning etc. This then means return to static text based websites which is fine for many use cases, but not for all.
Ideally, rich web apps which actually need all the CSS/APIs could use them, while the text-based sites would keep custom styling/JS to minimum (which would allow reasonable user-defined styling). But it's impossible to force this dichotomy in practice.