The humble typeface is not as humble as you might think, especially on a Mac. Fonts look more like plug-ins than you imagine, and because they are loaded by macOS and the applications that run on them, a bad font can have unintended consequences.
These consequences can range from relatively benign, such as the incorrect display of certain characters, to something more awkward, such as the refusal to load applications or functions that no longer worked as before. Faulty fonts are rarely the cause of Mac OS problems, but if you have problems that do not make much sense, checking them out is a first step in troubleshooting.
Fortunately, Apple includes a tool to install, validate and resolve font issues with each Mac. The next time your Mac will behave badly and you run out of ideas, validating your fonts is a good troubleshooting step.
How to validate fonts on a Mac
To start, open the Font Book application. You will find it in your Applications folder. (Open the Finder and click Go> Applications to open the folder.) Once opened, select all your fonts by clicking one of them and pressing Cmd + A.
Right-click one of the fonts and click "Validate Fonts."
Once the validation is complete, a new window will appear confirming if any of your faults is damaged. A green icon indicates that everything is fine, a yellow icon indicates warnings about the font, and a red icon indicates that the validation failed.
How to remove damaged or duplicate fonts
If any of your fonts is actually damaged, check the corresponding boxes and then click on the "Delete Options" button.
The selected fonts will be removed from your system. If this is actually necessary, restart your Mac once the fonts are removed.