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.
Endless waves, 3 lives, a boss every third wave.
Read Patrol 88 as a Kepung Shelf game first and a Kepung Shelf game second. The shape explains how the session will end; the group explains what it will ask of you before that.
Difficulty sits at Medium and the player count at 1. Those two numbers decide more about whether you will enjoy it than the theme does.
If you are choosing between this and something similar, start here when you have Medium energy and elsewhere when you do not. Patrol 88 does not pretend to be more than it is.