bolosaur - they might use custom Javascript to resize the images in the engine. I suppose we could take a similar approach but for large games it can affect the startup time. It's hard for us to take a general approach that works for all games - custom-coded games like BrowserQuest can use specific hacks to make things work as they need and it doesn't matter if it wouldn't work well for different types of game.
Have you tried r92 by the way? Should improve the point sampling on WebKit browsers.