What is now high end desktop graphics hardware (gtx 670) will in a few years be common in laptops. The modern gtx 640M, for example (a mainstream graphics chip for a mid range laptop) has performance comparable to a mid-range 9600GT desktop dedicated gpu from 4 years ago, at one third the power consumption, or an 8800GT, a high end card from 6 years ago. So if trends hold, GTX 670 performance will be available in a mainstream laptop profile in 5 years.
Go even further - the Tegra 4 coming out this year will feature up to 64 shader cores on an APU package running at 600mhz, which beats out the Playstation 3's 24 shader units at 550mhz in the RSX handily (other architectural changes, such as the lack of dedicated VRAM for bandwidth and latency, come into play, so comparing the two just on shader cores alone isn't sufficient). But in practical terms, next generation tablets will have graphical capabilities that rival or surpass consoles that are still the market leaders.
So a web game developer, especially a very future-thinking one, might want to try to build a game engine modeled around matching performance of devices 5 years from now on the high end, rather than treating the current status quo as the performance they should expect when web games using webgl really take off. Using a 670 or some comparable high end card to create demos that truly test the bounds of performance in these technologies now iron out the wrinkles 5 years from now when everyone's tablet is boasting similar performance.