Further, the drivers have to be more permissive than necessary. It's not an option for a GPU vendor to ship a driver update that would break Doom/Quake/Dota/etc

Further, the drivers have to be more permissive than necessary. It's not an option for a GPU vendor to ship a driver update that would break Doom/Quake/Dota/etc. Ideally games would ship only shaders that are strictly valid GLSL but that's not quite the case. If you're developing GL, do everyone a favor and start using the official GLSL validator in your build scripts.