Slow, physical paintballs.
One hit and you are out.
Paintballs drop and slow down, so lead moving targets. Crouch behind a low bunker and you are completely hidden. When you are hit the camera rises to show where you fell; click to watch someone else.
Full screen on an iPhone: Safari cannot do it for a web page, but an installed web app can. Tap the Share button, choose Add to Home Screen, then start Paintball from its new icon.
A body, nothing more: the same size and the same game. Bots are a mix.
Fullscreen also keeps Ctrl+W (crouch + forward) from closing the tab. In a window, Edge shows a "leave this site?" question instead; or crouch with C.Fullscreen hides the browser's bars while you play, where the browser allows it. A phone plays sideways.