CSS: “font-family”: X is an incorrect operator.

A CSS syntax error in your font-family declaration is using an invalid character or operator where the validator expects a comma-separated list of font names.

The font-family property expects font names separated by commas. A common mistake is using operators like +, =, or / between font names, or copying malformed font names from URLs (like Google Fonts link URLs where + replaces spaces). Font names with spaces must be wrapped in quotes.

For example, font-family: Open+Sans is invalid because + is not a valid CSS operator in this context. The correct form is font-family: "Open Sans". Similarly, using font-family: Arial / Helvetica is invalid — a comma should separate fallback fonts.

Invalid Example

<p style="font-family: Open+Sans, sans-serif;">Hello</p>

Fixed Example

<p style="font-family: 'Open Sans', sans-serif;">Hello</p>

Double-check that your font names use commas as separators, quotes around names with spaces, and no stray characters like +, /, or =.

Learn more:

MDN Reference

font-family - CSS: Cascading Style Sheets

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MDN Reference

CSS values and units

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