PTetris/HTML was born to celebrate the coming 10-year anniversary of PTetris, the same Tetris-clone implemented as a Java applet and application. Play PTetriS here, with a Java-enabled browser.
Please note that the PTetriS games are not feature rich (e.g. they don't count the score and they don't show the next brick, and they don't have multiple levels). This is intentional, so they can work as a reference implementation of a correct Tetris game. By correct I mean that the board size is correct, bricks rotate in the correct direction, and the rotation pixel pattern of the bricks is correct (e.g. they don't jump up or down when changing the rotation phase). These properties are copied from the old game FUXOFT's Tetris2 (architecture: ZX Spectrum). There are many Tetris-clones available nowadays which are incorrect.