Firefox on Ubuntu 16.04 NVIDIA proprietary driver here. I searched for 1080p on YouTube and the first result is this harmless video https://www.youtube.com/watch?v=DQuhA5ZCV9M
It started at 720p, then I switched it to 1080p and it started playing at that resolution. However I didn't count the lines and I don't know if this is the kind of streaming you're writing about. Maybe Netflix? I can't check that.
I'm pretty sure YouTube doesn't support DRM at all. Premium originals (e.g. Mind Field) play in 4K for me on FreeBSD (for which no one has ever compiled Widevine or anything like that)