This page contains links to various resources that will help you improve your games.
If your game requires a lot of mouse movements it would be better for users who don't play in fullscreen to lock the mouse (confine it to the game area), and optionally show a custom pointer if the game requires it.
A game that does it well: GunMaster.io.
A game that would benefit from this: Cubes 2048.io.
The main issue that users who don't play in full screen complain about are accidental clicks outside the game, which most often will just lead them to another game.
This is done differently in different engines, so here are some links that may be useful to you: