Sudoku.

Every row, column and 3×3 box needs 1–9, with no repeats. Select a cell, then choose a number. Pencil mode keeps possible numbers; a correct entry removes that number from related pencil marks. Arrow keys navigate, 1–9 enter, N toggles pencil mode, and Delete clears. New puzzles have one solution; levels vary the number of clues, rather than guaranteeing a particular solving technique.

Your timer waits for the first board interaction, pauses when this tab is hidden, and stops on completion. Progress stays in this browser. Hints are recorded, and only games without hints update your best time.