Ultimate Boxing is a boxing game with the fundamentals in place: jabs, hooks, a block, and stamina that depletes when you throw.
That stamina is the whole game. Throwing constantly leaves you defenceless, so the good fighters are the ones who wait, block, and punish the gap after their opponent's combination.
Every punch costs stamina, and low stamina means you can't defend. Blocking and countering rather than throwing constantly is how good players manage it.
By landing clean head shots when their guard is down — typically in the gap right after their own combination.
Yes — it runs in the browser with nothing to install.