[css-properties-and-values] Added <length-percentage> computation

rules.

Author: shans

css-properties-values-api/Overview.bs

@@ -270,6 +270,14 @@ described in [[!css3-values]] and [[!css-cascade-4]] respectively. Otherwise:
 
 For <length> values, the computed value is the absolute length expressed in pixels.
 
+For <length-percentage> values, the computed value is one of the following:
+*   if the specified value contains only length units, the computed value is the absolute length
+    expressed in pixels.
+*   if the specified value contains only percentages, the computed value is a
+    simple percentage.
+*   otherwise, the computed value is a calc expression containing an absolute
+    length expressed in pixels, and a percentage value.
+
 For <custom-ident>, string, <color>, <image>, <url>, <integer>,
 <angle>, <time>, <resolution>, <transform-function> or "*" values, the
 computed value is identical to the specified value.