What about Haskell, which compiles everything - including the garbage collector - into a static binary? Or is it not possible to have GC-thread running with WebAssembly?
I can't remember if threading support is in yet, but it should be possible. The downside is that it will be a rather hefty binary the user has to download.
Eh, cut one tracking script or bloated PNG and you've saved a lot of download size.