[css-values] Add <length-percentage>/etc.

Author: tabatkins

css-values/Overview.bs

@@ -669,6 +669,33 @@ Percentages: the <<percentage>> type</h3>
 	or a value of the formatting context
 	(e.g., the width of a <a>containing block</a>).
 
+<h4 id='dimension-percentage'>
+Percentages Relative to Other Values</h4>
+
+	In some cases, <<percentage>>s are interpreted relative to a <a>dimension</a> or <<number>>,
+	such as a <<length>>.
+	When this happens, <<percentage>>s and <a>dimensions</a>/<<number>>s can combined in ''calc()'' expressions.
+
+	While such "relative" percentages are often indicated in prose,
+	they can also be indicated with an explicit grammar term,
+	one for each basic <a>dimension</a> type:
+
+	: <dfn>&lt;length-percentage></dfn>
+	:: Equivalent to <code class=prod>[ <<length>> | <<percentage>> ]</code>,
+		where the <<percentage>> will resolve to a <<length>>.
+	: <dfn>&lt;frequency-percentage></dfn>
+	:: Equivalent to <code class=prod>[ <<frequency>> | <<percentage>> ]</code>,
+		where the <<percentage>> will resolve to a <<frequency>>.
+	: <dfn>&lt;angle-percentage></dfn>
+	:: Equivalent to <code class=prod>[ <<angle>> | <<percentage>> ]</code>,
+		where the <<percentage>> will resolve to an <<angle>>.
+	: <dfn>&lt;time-percentage></dfn>
+	:: Equivalent to <code class=prod>[ <<time>> | <<percentage>> ]</code>,
+		where the <<percentage>> will resolve to a <<time>>.
+	: <dfn>&lt;number-percentage></dfn>
+	:: Equivalent to <code class=prod>[ <<number>> | <<percentage>> ]</code>,
+		where the <<percentage>> will resolve to a <<number>>.
+
 <h3 id='dimensions'>
 Numbers with Units: <a>dimensions</a></h3>