[Narew]

I don't remember where I read that and was not able to find it again. There is a web/desktop app (like zoom) that install a font when you install the app, and the web app check if this font is install to trigger the open in app popup.

[sethhochberg]

It’s a common enough technique that this surely isn’t the only example, but there was discussion here a while back about TeamViewer doing this to detect the presence and version of the client software when clicking a link to open a remote session:

https://news.ycombinator.com/item?id=32165103

In their case, the (shell of a) font file goes a little further and encodes the version of the teamviewer client that installed it