Pocket Sudoku

Two difficulties are graded by the reasoning they need, not by clue count: Easy boards can be finished by finding cells with only one digit left, while Tricky boards need hidden singles or box-and-line eliminations. There are 60 numbered boards per difficulty, identical for everyone, and an unlimited supply of random boards seeded from the clock.

Every board has exactly one solution and was machine-checked to be solvable by those techniques alone, with no guessing. Pencil notes, undo and redo, conflict highlighting, same-digit highlighting and a timer are all there. The hint never just reveals a digit: it names the technique (naked single, hidden single, locked candidates, naked or hidden pair) and explains why that cell must hold that digit, or clears a wrong entry first.

Progress, best unassisted times and the board you are in the middle of are kept in your browser. Hand-written HTML5 with no engine, no dependencies and no network requests.