You might be able to get a headless JVM running on the web using the LLVM backend in Graal and the WASM backend for LLVM. One thing that's missing seems to be

You might be able to get a headless JVM running on the web using the LLVM backend in Graal and the WASM backend for LLVM. One thing that's missing seems to be tight integration of the UI aspects, for anything significantly advanced you're going to at least be waiting for WebGPU, and even for ordinary examples cross platform consistency is non-existent.