For something like YouTube it can probably still be detected by timing. If the browser plays part A of a video, then when it is done immediately requests part B

For something like YouTube it can probably still be detected by timing. If the browser plays part A of a video, then when it is done immediately requests part B of the video, when there is supposed to be a 2 minute gap to play an ad, that can be detected on the server side. The client could try to get around that by doing things like telling the server it is playing at 2x, and then really playback at 1x, but then the server could send different content (pre-downsampled) depending on the playback rate requested.