Your browser does not support WebUSB which is required for the web flasher to function.
The web flasher uses WebUSB to flash a postmarketOS image to your device.
Put your device in fastboot mode and use the "Give access" button to allow the flasher to control it.
This is a list of all the devices that are currently supported in this flasher
Recommended builds for trying out postmarketOS
Untested experimental builds