Well, part of the problem is that web apps themselves are kind of alien on the web. The web is generally document-based. Web apps take the document format and try to turn it into something it’s not.
There’s a way to not do the JavaScript, but it doesn’t fix things being document-based and it can be argued that it makes other things worse in some respects.

I’m talking about WebAssembly. Basically, you can write your web app in HTML+CSS+Rust and then the Rust part is compiled to WebAssembly, which then takes the role that JavaScript would normally take. It does not have to be Rust, lots of languages can be compiled to WebAssembly, but Rust has the most mature ecosystem for that, as far as I’m aware.

In principle, it is also possible to use WebAssembly to render directly to a pixel buffer, but that’s really rather heavyweight and not terribly responsive, so not generally done, unless you implement a game¹ or similar.
Alright, so back to the document mangling approach. There’s various frameworks available for Rust. I’ve used Leptos so far. There’s also Dioxus and Yew and probably others.

Advantages:

• Don’t have to write JS.
• Can write Rust. Rust has some concepts that mesh really well with frontend dev, like the Result and Option types for error handling, which you can pass directly to your rendering stack and it can show either the data or the error (or nothing).
• Can use the same language in backend and frontend and therefore also get compile-time checks that the two work together.

Disadvantages:

• The ecosystem is young. You will find barely a fraction of the component libraries as you can find for JS.
• Rust also has concepts which don’t mesh well with frontend dev, like the whole memory management concept. Those frameworks bypass that or make use of it in clever ways, but things can be a bit peculiar or overly complex at times.
• WebAssembly is sent to the browser in one big blob, because it’s a compiled program. This means you get somewhat of a loading time when first loading the web app. There’s ways to mitigate that with “hydration” strategies, but yeah, still a thing.
• While JS is often minimized/uglified and therefore not readable anyways, WebAssembly makes that even more of a reality, because it is essentially assembly code that’s sent to the browser. It does still call the same APIs under the hood as JS does, so content blocking shouldn’t be affected, but yeah, can’t try to understand the code itself. This can also make debugging during development somewhat more painful.
• Well, and it’s also yet another web standard that browsers have to support. It doesn’t make browsers simpler in the sense that suckless would like.

I’ve listed a lot of disadvantages, so just to point out that, yes, to me, the advantages are absolutely worth it. But I can totally understand, if others see that differently.

¹) See, for example, Bevy and this UI example in particular.