Fix: Use px unit in @property/initial-value example

files/en-us/web/css/@property/initial-value/index.md

@@ -17,14 +17,16 @@ If required but missing or invalid, the entire `@property` rule is invalid and i
 initial-value: rebeccapurple;
 
 /* Set initial length value */
-initial-value: 2rem;
+initial-value: 16px;
 ```
 
 ### Values
 
 A value that matches the type specified in the {{cssxref("@property/syntax", "syntax")}} descriptor.
 For example, if `syntax` is `<color>`, then the `initial-value` must be a valid {{cssxref("color")}} value.
 
+If the value of the `syntax` descriptor is not the universal syntax definition, the `initial-value` descriptor has to be a [computationally independent](https://drafts.css-houdini.org/css-properties-values-api-1/#computationally-independent) value. This means the value can be converted into a computed value without depending on other values, except for "global" definitions independent of CSS. For example, `10px` is computationally independent—it doesn't change when converted to a computed value. `2in` is also valid, because `1in` is always equivalent to `96px`. However, `3em` is not valid, because the value of an `em` is dependent on the parent's {{cssxref("font-size")}}.
+
 ## Formal definition
 
 {{cssinfo}}