If you have five spare minutes, Patrol 88 will fill them. Everything happens on one screen with 3 lives, bosses with 56 base HP every third wave, and the whole thing runs straight in the browser - no downloads, no sign-ups, no fuss.
Every third wave ends with a boss that soaks real damage and answers back. Regular enemies come in patterns - sine sweeps, dive bombs, wall formations - and each pattern has a spot where it cannot hurt you; finding those spots is the actual game.
Nothing here needs a tutorial or a wiki. Controls are one gesture, the goal is one sentence, and the difficulty comes from execution rather than homework. That is exactly what a browser game should be.