GPUs are just as power hungry as CPUs (keccak/sha3/sha2/cryptonight; it's going to draw more power). Monetizing your user base via any type of proof-of-work at this point is just plain stealing electricity IMHO, unless your user base is an ASIC farm or working some unheard-of cryptocoin with a low mining difficulty.

I ran 16 GPUs on Bitcoin for 18-24 months when it made sense to do so. I pool hopped to game rewards and had failover bitcoind solo nodes for when pools got DDoS'd. There was a time when it made sense, but ultimately GPUs got priced out.

The people who are chasing this dream are quite literally 6 or 7 years late to the profit margin vs. electrical efficiency equation in terms of Bitcoin and maybe 18 months too late for Monero. That's the evolution of all POW mining, electrical efficiency.

There is an old github repo on bitcoin browser mining from the era before coinhive. I remember it had something about using shaders and webgl to mine with the GPU.