What's the problem that this fixes?
You're right, there isn't yet here we are. Where I need to download and run a React program every time I want to read an article.
(IIUC, there was a proposal in Firefox decades ago to make the engine into several flexible modules and a page could declare which modules it depended upon, then the browser would either cache them and use them for multiple sites or already have them builtin. You'd get the best of both worlds: rich and expressive pages without the frequently-paid cost of poly-filling the gap between how the developer wants the render engine to work and the actual implementation of the render engine.
Sadly, I suspect the actual complexity to implement would have made for a worse overall situation than what we have now).
Where is the security? Chrome last I checked had eight actively exploited zero-days last year, which is laughably bad compared to the other operating system I use. Perhaps if the modern web was simpler, a browser would be easier to implement, and more time could be spent on making it not a raging security dumpster fire? But it ain't, it's complicated.
How does one even setup user agent stylesheets? What could that be but yet more complexity? Meanwhile, I'll use w3m and amfora and if it's a broken page that mandates Flash, JavaScript, whatever, I most likely won't bother launching a "heavyweight champion" browser. The CPU fans will last longer that way.
I still don't understand why it's utility is being questioned, when the entire category clearly has a user base.