Can I use it offline?

Yes, as long as you have added it to the Home Screen.

There is no sound.

Your device's "Silent Switch" should be "Off." If the sound is still not working, please try to restart the app/add another icon on Home Screen/reboot the device.

The performance is too slow/laggy.

Please ensure iOS's system-wide "Low Power Mode" is disabled. The battery icon is yellow if it is enabled.

The Select/Start buttons are overlapped by the Home Bar.

That's okay. You can touch them as usual. If the buttons are not working, it means the game does not use the Select/Start.

How do I save my progress?

Just save in the game and wait a few seconds. An auto-saving banner will appear, and your saved data will automatically be stored in the web app's local storage. On iOS, you must add the site to Home Screen first, and your data will be deleted when the Home Screen icon is removed.

Do I have to backup/restore the savegame in the menu?

You don't need to backup/restore the savegame manually since auto-saving is present, just save in the game and wait a few seconds. To prevent data loss caused by accidental deletion of the Home Screen icon or damaged device, you may connect to dropbox to enable automatic cloud backup.

After importing the savegame, it takes me back to the main menu.

It is expected behavior, the save data was imported, and you can continue playing by loading the game file again.

It can't load any files.

Only iOS >= 14.7 is supported. Please update your OS first.

How to blow on the microphone?

Pressing the 'R' button will emulate a blow on the microphone. It may be helpful in playing some games.