[css-properties-values-api] Specify that calc expressions get reduced

during computation for <number> and <percentage> values.
w3c#69

Author: shans

css-properties-values-api/Overview.bs

@@ -209,9 +209,14 @@ described in [[!css3-values]] and [[!css-cascade-4]] respectively. Otherwise:
 
 For <length> values, the computed value is the absolute length expressed in pixels.
 
-For <number>, <percentage>, <custom-ident>, string or "*" values, the
+For <custom-ident>, string or "*" values, the
 computed value is identical to the specified value.
 
+For <number> and <percentage> values which are not calc expressions, the
+computed value is identical to the specified value. Calc expressions that are
+<number> and <percentage> values get reduced during computation to simple
+numbers and percentages respectively.
+
 For values with a syntax that references a CSS property, the computed value
 is defined by that property.