Donka Puzzle Co. makes puzzles that work properly, load quickly, and don't ask you for anything before you can start playing.
A free online Sudoku game with two modes. The Daily Challenge is one shared puzzle per calendar day — the same board for every visitor, resetting at midnight, with a streak counter for consecutive days completed. Practice mode is unlimited, across Easy, Medium, Hard and Expert.
Alongside the game there's a set of strategy guides covering everything from the basic rule to X-Wing, each one worked through with grid diagrams rather than described in the abstract.
Every board is generated in your browser and then checked by a solver before you see it. Two things are verified: that the puzzle has exactly one solution, and that it can be reached by logical deduction alone. Guessing is never required, at any difficulty.
The difficulty rating comes from that same solver. It works through the puzzle using only techniques a person would use, in order of difficulty, and the puzzle is rated by the hardest technique it turned out to need. That's why an Easy puzzle never demands pencil marks and an Expert one reliably does — the rating describes the reasoning required, not the number of starting clues.
The Daily Challenge is generated from a fixed seed derived from the date, which is how everyone gets the same board without any of it being stored on a server.
The whole game is a single HTML file. No frameworks, no build step, no analytics platform, no tracking scripts beyond the advertising described in the privacy policy. It loads in well under a second on a slow connection and works offline once loaded.
Your streak, your progress on the current daily puzzle, and the name you enter are stored in your own browser using local storage. They never leave your device and there's no account system, because there's nothing to have an account for.
Sudoku was a pencil-and-paper puzzle for decades before it was a website, and most people's instinct when solving is still to pencil in candidates and cross them out. The ruled paper, spiral binding and pencil-marked notes are there because that's what the game actually is — and because a grid of digits deserves a better setting than a white rectangle.
Corrections, bug reports, or a puzzle that behaved strangely — the contact page has the address. Reports of a puzzle with no valid solution are particularly welcome; they shouldn't be possible, and if one gets through I'd like to know.