Comment by om2

Fetch API is a pretty recent addition to the web platform. Back in the day, you could absolutely embed images of stylesheets from ftp: URLs. You could even use it with XMLHttpRequest (predecessor of Fetch). Even further back, gopher: was integrated with the web. URL schemes were invented for the web with the idea that http: is not the only one. These other protocols were really part of the web until they weren’t.