There are myriad problems with icon fonts. I won’t rehash those again here but I did a section on this specific topic in The Scoville Scale of Web Font Loading Opinions. A bunch of people have written about this before:
The main point that I think hasn’t really been communicated enough is that icon fonts exist in a place that would seem to be outside of the web standards mainstream. Specifically, the font loading poster child—the
font-display descriptor—has no valid value that is compatible with icon fonts.
When you load an icon font, you often never want the fallback text to render. It isn’t a typical Flash of Unstyled Text (FOUT) scenario. If the fallback text for an icon font renders, who knows what you might see.
If the icon font uses glyphs mapped to the Private Use Area, you may see emoji characters (this is a common failure scenario).
If the icon font uses ligatures, you could see the ligature text (which is better, but often also not handled very well).
Some browsers (Firefox) show a TOFU character as fallback.
An astute web font loading reader might reach for the easiest tool in the toolbox to solve font loading issues: the
font-display descriptor. However there are no good values for
font-display that will offer invisible text. In fact, a few of the values will be quite bad.
font-display: optionalon your icon font means that it will only render on repeat views and renders fallback text on empty cache views: Very Bad ?
font-display: swapon your icon font means that it will render fallback text immediately while waiting for the web font to finish loading: Bad ?
font-display: fallbackwill only render the icon font if it loads within a short (usually 3 second) time period: Not great ?
font-display: blockis the default behavior and will use invisible text for up to 3 seconds and show fallback text until the web font load completes: The best option but still not good ?
To workaround these issues, you could use the CSS Font Loading API to force invisible text until the icon font has successfully loaded. Or, perhaps more in line with the web standards mainstream, you could make a better investment and convert your icons to use SVG instead.