CSS: “margin-bottom”: X is not a “margin-bottom” value.

An invalid value was assigned to the margin-bottom CSS property, and the W3C validator rejected it because it does not match any accepted syntax for that property.

The margin-bottom property accepts a length (e.g., 10px, 2em), a percentage (e.g., 5%), auto, or the global keywords inherit, initial, revert, and unset. The validator raises this error when the value is misspelled, uses a wrong unit, is missing a unit entirely, or contains an otherwise unrecognized token.

Common causes:

• A bare number without a unit, like margin-bottom: 10 instead of margin-bottom: 10px. Zero is the only length value that does not require a unit.
• A typo in the unit or keyword, like margin-bottom: 10 px (with a space) or margin-bottom: auто (mixed character encodings).
• An invalid keyword, like margin-bottom: none. The margin-bottom property does not accept none.

Example with the error

<p style="margin-bottom: 10">This paragraph has an invalid margin.</p>

The value 10 is not valid because it lacks a unit.

Fixed example

<p style="margin-bottom: 10px">This paragraph has a valid margin.</p>

Adding a recognized unit like px, em, rem, or % fixes the error. If the intended value is zero, margin-bottom: 0 is valid without a unit.

Learn more:

MDN Reference

CSS margin-bottom property

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MDN Reference

CSS values and units

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MDN Reference

CSS margin

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