I am so happy right now! As of sha 4972fdb350fe79e18b0413e74028cd9b9803f96b (1 April), you can build Qt 6 for WebAssembly!
Not only that, because QtQuick3d in Qt 6 now supports OpenGL ES2/3, it will run in a web browser!
Here is a video of the helloquick3d example, running in Firefox: [edit] running at 62 fps
if that does not work, try this link:
Now I can get on with my life and stop working on build system stuff!