Why is my browser so energy inefficient when playing YouTube videos?
I thought this would be a problem long gone in 2020 but it's still not. I had it for decades like everybody else I guess: when I'm playing a video on YouTube using any browser (FF/Chrome) my laptop spinner is going crazy and I can feel the heat. If I download the same video with YouTubeDL and watch it with VLC/MPC-HC or any other player the laptop stays cool.
Any idea why browsers are still so bad at using the specialized video decoding hardware built-in processors for a decade?
It is using hardware decoding in the browser. Try playing a YouTube video on an old computer with no hardware decode or an OS with it lacking like Haiku.
If you are on a mac try safari, for some reason safari is far more efficient with youtube.