https://techcrunch.com/2017/08/15/google-allo-arrives-on-web-only-pairs-with-android-phones-at-launch/ :
To get started, you have to launch the web client in Google’s Chrome browser. You’ll then need to open Allo on your Android phone and then make sure you see “Allo for web” in the mobile app’s menu. From here, you’ll select “Scan QR code.”

With your phone, you scan the unique code that’s displayed on the Allo website, which will pair your app with the computer.

Because the web version merely mirrors what’s on your mobile device, if your phone runs out of battery and dies, you’ll no longer be able to use the Allo web app.

(et quand ils disent "Google Chrome", ce n'est pas un exemple : leur appli "web" ne fonctionne que sous Chrome).