CSS: “font”: X is not a “font-family” value.

The CSS font shorthand property has a specific syntax defined in the CSS specification. At minimum, it requires both a font-size and a font-family value. Optionally, you can prepend font-style, font-variant, and font-weight, and you can append a line-height value after the font-size using a slash separator. The full syntax looks like this:

font: [font-style] [font-variant] [font-weight] font-size[/line-height] font-family;

When the W3C validator reports that a value “is not a font-family value,” it means the parser reached a point in the font declaration where it expected to find a font family name but instead found something it couldn’t interpret as one. This often happens in two scenarios:

1. Using font when you meant a specific property — For example, writing font: 300 when you only intended to set the font weight. The validator tries to parse 300 as a complete font value, and since there’s no font-size or font-family, it fails.
2. Incomplete font shorthand — Providing some values but forgetting the mandatory font-family at the end, such as font: 300 16px without a family name.

This matters because browsers may ignore an invalid font declaration entirely, causing your text to render with default or inherited styles instead of what you intended. Keeping your CSS valid also ensures consistent behavior across different browsers and helps maintain clean, predictable stylesheets.

How to fix it:

• If you only need to set a single font-related property, use the specific property (font-weight, font-size, font-style, font-variant, or font-family) instead of the font shorthand.
• If you want to use the font shorthand, make sure you include at least font-size and font-family, and that all values appear in the correct order.
• Remember that the font shorthand resets any omitted font sub-properties to their initial values, so use it deliberately.

Examples

Incorrect: Using font to set only the weight

<p style="font: 300;">This text has an invalid font declaration.</p>

The validator reports that 300 is not a valid font-family value because the font shorthand expects at least a font-size and font-family.

Correct: Using font-weight directly

<p style="font-weight: 300;">This text has a light font weight.</p>

Incorrect: Missing font-family in the shorthand

<p style="font: italic 300 16px;">This is also invalid.</p>

Even though font-style, font-weight, and font-size are all present, the required font-family is missing.

Correct: Complete font shorthand

<p style="font: italic 300 16px/1.5 'Helvetica', sans-serif;">This is valid.</p>

This includes all components in the correct order: font-style, font-weight, font-size/line-height, and font-family.

Correct: Minimal valid font shorthand

<p style="font: 16px sans-serif;">Only size and family — the minimum required.</p>

Correct: Using individual properties instead of the shorthand

<p style="font-style: italic; font-weight: 300; font-size: 16px; line-height: 1.5; font-family: 'Helvetica', sans-serif;">
  Each property set individually.
</p>

Using individual properties avoids the pitfalls of the shorthand and gives you explicit control without accidentally resetting other font sub-properties.

Learn more:

W3C Reference

CSS Fonts Module Level 4: the font shorthand property

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MDN Reference

MDN Web Docs: font - CSS shorthand property

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W3C Reference

CSS Fonts Module Level 3: font shorthand property

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MDN Reference

MDN Web Docs: font-weight

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W3C Reference

CSS Fonts Module Level 4: font-weight property

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