Offline
lisudoku also works offline! For the best experience you can install it as an app.
iOS/iPadOS
Share > Add to Home Screen (source):max_bytes(150000):strip_icc():format(webp)/MakeSafariBookmark-9eff4fc5264546418637785165ae8541.jpg)
Android Chrome
Click "Add lisudoku to Home Screen" on the bottom (source)
Desktop Chrome/Edge
Click the icon in the URL bar
More
You can find full instructions here.