I associate this with eg. characters missing from a typeface, but I thought that was what Font Awesome was designed to solve. In case it’s relevant, I do have the following in my custom CSS (though removing it didn’t seem to solve the problem):
@frostedechoes thanks so much for this explainer. I’ve managed to get this working on my site, but — given that it’s something of a pain to create (light & dark theme versions of) CSS classes for each SVG, I may wait on a response from Manton before I go ahead and do the work of implementing this site-wide.
Sorry, I’m very late following up on this… Turns out I had broken the CDN where we were storing Font Awesome files. I’ve updated the Font Awesome plug-in (and the Gateway plug-in) to embed the fonts directly on your blog.
You should see an Upgrade button for those plug-ins that will install the fix.