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ValidateHTML

Unknown CSS Property

An unknown property error means the CSS property name doesn't exist in the specification. This is usually a typo (e.g., 'colr' instead of 'color'), a vendor-prefixed property used without the prefix, or a made-up property name.

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16.3%
of sites tested

How common is this error?

We ran this validator over the home page of the Tranco top 5,000 websites in August 2026. 433 of the 2,656 sites we could reach hit it. That makes it the 2nd most common CSS problem we found, across 4,588 occurrences. See the full study.

Most frequent wording: Unknown property `ms-flex-preferred-size`

Why It Matters

The browser ignores the entire declaration. Unlike invalid values, unknown properties are easier to catch because the property name itself is wrong. But they can hide behind vendor prefixes and preprocessor variables.

Common Causes

  • Simple spelling mistakes like colr, font-weigth, or heigth that no longer match any real property.
  • Using British spelling: background-colour and centre are not valid CSS keywords or properties.
  • Inventing a plausible-sounding property such as text-size that does not exist in the spec.

Code Examples

Bad CSS
div {
  colr: red;              /* typo */
  font-weigth: bold;      /* typo */
  background-colour: #fff; /* British spelling */
  text-size: 16px;         /* not a property */
}
Good CSS
div {
  color: red;
  font-weight: bold;
  background-color: #fff;
  font-size: 16px;
}

How to Fix

  • 1Double-check property names. Common typos: colr, font-weigth, backgroud, heigth, widht.
  • 2background-colour is not valid CSS. Use background-color (American spelling).
  • 3text-size doesn't exist. Use font-size for text sizing.
  • 4If using vendor prefixes (-webkit-, -moz-), also include the standard property.

Frequently Asked Questions

What is the difference between an unknown property and an invalid value?
An unknown property means the name on the left of the colon does not exist (colr). An invalid value means the property is real but the value is wrong (color: redd). Both cause the declaration to be dropped, but for different reasons.
Will an unknown property break the rest of my rule?
No. The browser discards only the unrecognized declaration and keeps applying every other valid line in the same block. The cost is the missing style, not a broken rule.
Are custom properties treated as unknown?
No. Custom properties (CSS variables) begin with a double-dash prefix and are always valid to declare. Only standard property names that the browser does not recognize are flagged as unknown.

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