I also get a flickery mess, on Linux + Firefox 81 with Haswell Intel Graphics. I suspect that there is some sort of uninitialised buffer/variable somewhere, and different drivers handle it differently.
I have tried the page again, and now it actually works as intended. I suspect the shader got updated/fixed?
Works fine for me (Firefox 82 (beta), Linux, AMD RX 5700 XT, Mesa 20.2.0, llvm 11).
It's working for me on Ubuntu 20.04, with PowerColor Vega 56, and Firefox 81.