I was finally able to demonstrate that the problem is on the client side. The client machine is running Windows 10, with some extra customer-specific security settings. Those security settings are what prevent the FontAwesome font from being downloaded. I have not yet determined which security setting is the root cause, but that is out of scope. There is no problem on the server side with Moodle or the Essential theme.
Thanks for helping me gather the evidence.
Links that I found while investigating, which may be useful to others.
- https://github.com/FortAwesome/Font-Awesome/issues/8825
- https://github.com/FortAwesome/Font-Awesome/issues/8472
- https://github.com/FortAwesome/Font-Awesome/wiki/Troubleshooting#fonts-not-rendering-properly-running-windows-10
- https://docs.microsoft.com/en-us/windows/security/threat-protection/block-untrusted-fonts-in-enterprise#Turn_on_and_use_the_Blocking_untrusted_fonts_feature
- https://blogs.technet.microsoft.com/secguide/2017/06/15/dropping-the-untrusted-font-blocking-setting
- https://github.com/FortAwesome/Font-Awesome/wiki/Troubleshooting#im-hosting-fonts-on-my-server-and-icons-dont-show-up
- https://github.com/FortAwesome/Font-Awesome/issues/6454
Hmm, I can change the subject of this reply, but it does not percolate to the whole thread. Alas!