This optimization overrides the font_display property for all page fonts. Changing the font_display value can prevent font loading from being render-blocking.
The allowable values for this option are the same as those for CSS.
The default value is fallback which means that the font is not used unless it is already cached by the browser. Fonts are often in the browser cache – especially for Google Fonts – and will definitely be cached when a user visits subsequent pages on your site.
This option allows the user to correct poorly implemented CSS without a code change.
If an icon font is used on the page, then this option could break that functionality. Use this option in conjunction with font_map to fix broken functionality or use the individual icons instead.