Well here are a few reasons why I don't like or will like Fullscreen.
1. It makes my pc the whole game which means I can't use my discord or lookup anything or multitask.
2. If something happens in game and crashes it stops my whole browser from working and if Fullscreen its even worse because some will just freeze my display instead.
3. I should have a choice in if I want to play in Fullscreen or not? I play your game not because "I have to" I do it out of enjoyment and if I am not comfortable or enjoying my display being changed to suit your game and that is the only way I can play how is that fair to the player?
4. Fullscreen is a poor excuse for most for not having to "Optimize" or even put effort into making a proper Webgl game.
5. I use my laptop atm and Full screened games use more ram in the browser which I am limited to on my laptop.
Do I really need an excuse to not use Fullscreen ? It should be the players choice.