Unfortunately not - they have to be square power-of-two. The problem is the lowest common denominator is mobile, which tend to have much more limited hardware, and get this limitation. This then "leaks" over to desktop machines, which don't have the limitation, because technologies like WebGL only support the lowest-common-denominator as well, specifically to maintain compatibility with mobile. So now your desktop machine using WebGL has to use a square power-of-two texture for good tiling, even though the hardware powering it is 99% likely to be able to tile any size texture perfectly. I tried to press the WebGL standards committee to add support for the desktop extension when it's available, but they didn't want to add it. I think it was mainly because WebGL is still relatively new, and they don't want to add features which don't work everywhere yet.