Icons with shading won't work well for the favicon nor will anything that is hard to make transparent outlines or colors that will be hard to see for dark or light theme tabs and highly detailed icons just plain don't scale. This is the 4 options at 32x32 which is the "Larger" usual icon size (can be shrunk to 16x16 for some users/browsers):
View attachment 2917
(edit: I labeled them wrong, it's 1,3,2,4 I apparently can't copy and paste)
Here's 2 simple icons that both look decent at high resolutions, but look how messed up the more intricate lines are on the one when scaled favicon size
View attachment 2918
Keep them simple and they can scale, get intricate and make someone's life hell. Thankfully I don't draw graphics, I just toss them into web apps for a living.
Dart