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Add constraint for initial value in @property rule #38321

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Add constraint for initial value in @property rule #38321

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3 changes: 3 additions & 0 deletions files/en-us/web/css/@property/index.md
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Expand Up @@ -42,6 +42,9 @@ The following conditions must be met for the `@property` rule to be valid:
If either is missing, the entire `@property` rule is invalid and ignored.
- The {{cssxref("@property/initial-value","initial-value")}} descriptor is optional if the value of the `syntax` descriptor is the universal syntax definition (that is, `syntax: "*"`).
If the `initial-value` descriptor is required but omitted, the entire `@property` rule is invalid and ignored.
- The {{cssxref("@property/initial-value","initial-value")}} descriptor has to be a [computationally independent value](https://www.w3.org/TR/css-properties-values-api-1/#computationally-independent)
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- The {{cssxref("@property/initial-value","initial-value")}} descriptor has to be a [computationally independent value](https://www.w3.org/TR/css-properties-values-api-1/#computationally-independent)
- If the value of the `syntax` descriptor is not the universal syntax definition, the {{cssxref("@property/initial-value","initial-value")}} descriptor has to be a [computationally independent](https://www.w3.org/TR/css-properties-values-api-1/#computationally-independent) value.

if the value of the `syntax` descriptor is not the universal syntax definition (that is, `syntax: "*"`).
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if the value of the `syntax` descriptor is not the universal syntax definition (that is, `syntax: "*"`).
This means the value can be converted into a computed value without depending on other values, except for "global" definitions independent of CSS.

This excludes relative units like `rem`.
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This excludes relative units like `rem`.
For example, a pixel value like `10px` is computationally independent and therefore valid — it doesn't change when converted to a computed value. An inch value like `2in` is also valid, because `1in` is always equivalent to `96px`. However, an em value like `3em` is not valid, because the value of an em is dependent on the parent's {{cssxref("font-size")}}.

- Unknown descriptors are invalid and ignored, but do not invalidate the `@property` rule.

## Formal syntax
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