CSS: “line-height”: X negative values are not allowed.

Negative values for the CSS line-height property are invalid and browsers reject them.

The line-height property sets the height of each line box, so a negative value has no meaning and violates the CSS specification. Valid values are normal, a unitless multiplier like 1.5, a length like 24px or 1.5em, or a percentage like 150%. A unitless number is usually the best choice, since it scales with the element's font size and avoids the inheritance quirks that fixed lengths can cause.

This error usually appears when line-height is set inline through the style attribute, where the W3C validator catches it during HTML validation. A common cause is a calculation in a template or script that produces a negative number and writes it into the markup.

To fix it, replace the negative value with a positive one, or use normal for the browser's default spacing. Tighter lines are fine as long as the value stays positive, such as line-height: 0.9.

HTML examples

Invalid: negative line-height value

<p style="line-height: -0.5em;">
  Paragraph text
</p>

Valid: positive value or normal

<!-- Unitless multiplier, scales with font size -->
<p style="line-height: 1.5;">
  Paragraph text
</p>

<!-- Browser default spacing -->
<p style="line-height: normal;">
  Paragraph text
</p>

Learn more:

MDN Reference

CSS line-height property

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MDN Reference

CSS values and units

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