I assume you meant the current frame? The problem, at least on Apple platforms, is that the video decoding and the compositor might each be in another process, making it difficult for the browser to get access to the frames :(
Wait a second. The compositor is implemented by the browser (the page compositor). So it is known that browsers are capable to playback Full-HD video. It is also known that these days the compositor is hardware accelerated. So this has to mean that, if you are decoding video in a separate process, that there is a fast way to share that with the compositor. And because the compositor is also sharing with the WebGL context, that has to mean that there has to be a fast way to share video with WebGL in principle.