Tatask keeps a complete copy of your task list on every device you use. When you add or tick off a task, it's saved on that device straight away, and then sent to your other devices in the background. If there's no connection, nothing stops: the change waits and goes out when you're back online.
Why it's built this way
A to-do list is something you reach for at odd moments: on the train, in a basement car park, on a plane, in a café with bad wifi. If the app needs a server to answer before it does anything, those are exactly the moments it fails.
So both the web app and the mobile apps are built offline-first. Each one runs on a local database in the browser or on the phone, and talks to your online account to sync. For the technically curious, I've written up how Tatask works offline, including the database behind it.
Sync between devices
While your devices are online, changes travel between them within a few seconds. Tick a task off on your phone and it's ticked off on your laptop. Delete a task on the web and it disappears from your phone too.
When a device has been offline, it catches up the next time it connects, or when you come back to the app. If you changed the same task in two places, the latest edit wins.
Web, iPhone and Android
The same account works in the web app, on iPhone and Android. Plan the week at a desk on a big screen with the column view, then work through Today on your phone. I wrote about keeping one list across every device and why it matters.
One subscription covers all of them, wherever you bought it.
Safe on shared computers
When you sign out of the web app, the local copy of your tasks is removed from that browser. If you use Tatask on a shared or work computer, sign out when you're done and nothing is left behind.
What sync doesn't include
Theme and some display preferences are kept per device, so your phone can be dark while your laptop is light.