Karnage.io is one of the best shooter io games in my opinion. You play as a block-type character, on a map surrounded by lava. It's a big battle between all of the connected players.
To play, you can move around with WASD, and you can fire your weapon with left click.
The strategy in this game is very similar to games like Slay.one... You want to use coverage as best you can so that you always have an advantage over other players.