Looks like most people have misunderstanding: they think that WebAssembly is the only way to run their favourite language in the browser. However, there's another way that exists for decades: simply use compiler of their favourite language that produces JavaScript! As for Java, GWT exists almost 20 years. Another example I know of is Brython which runs Python on top of JavaScript. However, looks like Java developers don't like to write websites, even while GWT is there.

And according to my experience, WebAssembly is not a game changer: it does not provide extra performance here, neither does it allow to get smaller binaries. I'm a developer of another Java-to-JavaScript compiler, TeaVM. I recently compared performance of JavaScript and WebAssembly targets, and in one particular case WebAssembly is slower: https://teavm.hashnode.dev/comparing-teavm-with-kotlinjs-pef.... There's another example where WebAssembly is only slighly better: https://teavm.org/gallery/jbox2d/index.html. In both cases WebAssembly binary is huge compared to JavaScript. On one hand this can be because of I don't target Wasm GC, so binaries get bloated because of need to maintain shadow stack. On another hand, there's Google Closure Compiler, which produces Wasm GC binaries, and from what I heard of it, WebAssembly binaries still don't win in their size.