I mean something more suitable than the HTML+JS+CSS combination. HTML is a document markup language, not a UI definition language. CSS mixes layout with styling, which are largely orthogonal. (One should be able to specify a UI independently from styling/theming.) A programming language like Javascript shouldn’t be needed for building a UI, in the majority of cases. Most UI components and behaviors should be standard (built into browsers, or whatever is used as the UI runtime) and declarative.

So basically you’re saying someone should make mui or any other component library except built into browser. Why is that an improvement?

It would be more than a component library, but yes, more akin to traditional GUI frameworks in spirits than the HTML+CSS approach. The improvement is that it would dramatically reduce the effort needed to implement and maintain web application, make app behaviors more consistent for users, and make adherence to accessibility and other best practices the default, rather than something developers have to actively put significant effort into.