What "online" actually means here
A lot of "play online" pages mean a download page with a play button on it, or a game that needs an account before it will remember anything. This one runs in the browser and stores your progress on your own device. Your streak, your times, your half-finished board and your settings live in this browser and are sent nowhere.
That has a practical consequence worth knowing: clearing your site data clears your record. There is an export in your settings that writes everything to a file, which is also how you move to a new phone.
It keeps working without a connection
After the first visit the whole thing is cached, today’s puzzles included. On a train, on a plane, in a lift - the board still loads and still saves. Nothing about solving requires the network, because the checking already happened before the board was published.
You can add it to your home screen if you would rather it opened like an app. It is the same site either way; there is no separate download and no store listing.
Controls, on whatever you are holding
On a phone, tap a cell to cycle it: empty, crossed out, sentinel. Drag to sweep a run of crosses across a row, which is most of what you do on a large board. On a desktop, the arrow keys move and the space bar cycles; right-click steps backwards through the same cycle if you overshoot.
The board scales to the space it has rather than to a fixed size, so a 7×7 on a small phone and an 11×11 on a wide monitor both fill the screen properly. There is an assists panel that will mark rows as they complete, if you like that sort of help, and it is off by default because plenty of people do not.
If you are new to it
Place one marker in every row, column and coloured region, and never let two touch - including diagonally. That is the entire rule set.
The first board you meet is deliberately gentle. There is a full guide with worked examples of each deduction if you want the theory first, and a hint button on the board if you would rather learn by getting stuck and then being shown why.
Common questions
Do I need an account?
No. There is nothing to register and no email to give. Your streak and statistics are stored in this browser, on this device, and are not sent anywhere.
Will it work without an internet connection?
Yes, after the first visit. The site and the day’s puzzles are cached, so the board loads and saves normally on a plane or underground.
Can I play on a phone?
Yes. Tap a cell to cycle it, and drag to sweep a run of crosses across a row. The board sizes itself to the screen rather than shrinking a fixed layout.