If you force h.264, then the fans shouldn't ramp up.
That means using Safari in macOS 10.x or else a different browser with a plug-in. (Safari in macOS 11 switches to VP9, and Chrome by default uses VP9.)
I'll have to try this because viewing online video causes the fans on my Core 2 Duo, i5 and even i7 machines to make a good attempt to rival the "wind tunnel" of my Quicksilver.