Hardware accelerated video decode is supported in most platforms like Windows, Mac OSX, ChromeOS, etc, to improve Video playback performance. On mobile platforms, most of the videos are hardware decoded now days. With limits resources on mobile platform, it will be quite inefficient if we tried to read the memory from GPU back to CPU when drawing video to Canvas. When the Canvas is hardware accelerated, there is a possibility to do a GPU-GPU texture copy instead of CPU readback when drawing video to Canvas. There is another patch in Chromium that needed to combine with this patch to enable this feature for Canvas2D painting on android platform.
We believe that this is true for all Cocoa-based platforms. I'm not sure about GTK or WinCairo ports.