The classic HLSL compiler is infamous for being full of bugs (and slow, for that matter) so I wasn't surprised to see it come up here...

Fantastic seeing the author lay out all the steps involved in getting a renderdoc capture of a browser and what's involved in debugging shader issues in a webpage.

It's interesting that the issue only manifested on the geforce 40xx if it was an fxc issue though... wonder if it's fxc combined with a driver issue and not just one or the other?