Wolfenstein 3D is the game that made first-person shooters a genre. id Software released it in May 1992, and everything that followed — Doom, Quake, Half-Life — starts here: a maze of corridors seen through the eyes of B.J. Blazkowicz, an American spy shooting his way out of a Nazi castle, with a status bar showing his face getting progressively more beaten up.
There are six episodes of ten levels each, secret doors hidden in the walls, treasure to collect, and a boss at the end of every episode. By modern standards the maps are flat — no stairs, no looking up or down — and that constraint is exactly why it runs perfectly in a browser tab thirty years later.
It is the game that popularised the genre and set its template — a first-person view, a weapon on screen, corridors and keys. Earlier experiments existed (Maze War, Catacomb 3-D by the same team), but Wolfenstein 3D in 1992 is where the modern first-person shooter starts.
Sixty in the original: six episodes of ten levels each, with a boss at the end of every episode, plus hidden secret levels.
Yes — this is a browser port that runs with nothing to install. It is a 1992 game, so it runs at full speed on any hardware.