YouTube videos don’t serve 4K on Safari. When they do, it’s over Chrome which is using 10W more powerr, which over entire world equals power use of a small country...
Kaby Lake/Coffee Lake have full hardware VP9 decoding, so all told you'll probably be using more energy playing H.264 in Chrome if you're on a Kaby Lake or Coffee Lake processor.
Yes, been using that. Problem is that YouTube quietly removed 1440p h264 videos too! Used to be my indicator that I can turn off h264ify for a minute to view 4k content...