WebFonts needs to be loaded for anonymous users. Unfortunately that is only possible if the squid caches are purged for all wikis on which the extension was deployed.
List is at https://translatewiki.net/w/i.php?title=User:Siebrand/WebFonts_deployment&oldid=3494565
According to Roan, the reactive purge script Mark used for the Vector switchover should be used.
RT ticket filed: https://rt.wikimedia.org/Ticket/Display.html?id=2122
Version: unspecified
Severity: normal