Vex 4 is a precision platformer that expects you to die a lot. Your stickman runs, jumps, slides, swims, wall-jumps and climbs through levels packed with buzzsaws, spikes, crushers and disappearing platforms, and the checkpoints are frequent because the deaths are constant.
The series has been a browser staple since 2011, and the fourth game added the act structure and a level editor. The controls are tight enough that every death feels like your mistake rather than the game's, which is exactly why people keep restarting.
Nine acts of several levels each, plus bonus challenge levels and a level editor for player-made stages.
Deliberately — it's a precision platformer built on trial and error. Checkpoints are frequent enough that dying costs seconds rather than progress, which is what makes the difficulty fair.
Yes — a 2D keyboard game that runs in the browser with nothing to install.