A recent Youtube Web update has added a canvas whenever the seek bar is visible, an HTML5 canvas pops up. This was not asked for and not needed. If you disable canvases for privacy, this will cause a horrific red bad to cover half the screen until you hide the seekbar. Canvases can be used for fingerprinting, which I’m sure Google is doing here.
Maybe, I don’t know if it was on purpose. But the red lines are the red-fade-to-pink effect of the progress bar I believe, and I have not found a need for such a feature, so they might be using this feature as an excuse to claim the need of canvas.
Sure sounds like it’s just an excuse, since it worked swimmingly before the canvas claim.