Slide every tile one way at a time. Two tiles carrying the same number merge into one carrying double, and a new 2 or 4 appears in a free square after any slide that moved something. A tile that has just merged is finished for that slide, so four 4s become two 8s rather than one 16.
Arrow keys or WASD to slide, Z to take one move back, N for a new game, M to mute, F for fullscreen.
Swipe anywhere on the board, or use the arrows under it. Undo takes one move back. Your game is saved on this device, so you can close the tab and come back to the same board.