It is not always necessary to wrap font family names around quotes but it is safer.

Noncompliant Code Example

font-family: Arial, MS Trebuchet, sans-serif;

Compliant Solution

font-family: 'Arial', 'MS Trebuchet', sans-serif;

Exceptions

Generic font family names (serif, sans-serif, cursive, fantasy and monospace) must not be quoted.

See

• CSS Tricks - font-family
• Unquoted font family names in CSS

stylelint Related Rules

• font-family-name-quotes