after reading your comment I checked again and realized that activity monitor has a column for %GPU as well under the CPU tab. So here's the eyeball average after watching the GitHub page for about a minute.
Firefox 88 (no extensions, private mode):
CPU% 55 (avg) 61 (peak)
GPU% 65 (avg) 77 (peak)
Safari (no extensions):
CPU% 33 (avg) 40 (peak)
GPU% 33 (avg) 40 (peak) <- same numbers for both CPU and GPU in safari.
Firefox 88 (no extensions, private mode):
CPU% 55 (avg) 61 (peak)
GPU% 65 (avg) 77 (peak)
Safari (no extensions):
CPU% 33 (avg) 40 (peak)
GPU% 33 (avg) 40 (peak) <- same numbers for both CPU and GPU in safari.