[css-values-4] Define/export combine.

Author: fantasai

css-values-4/Overview.bs

@@ -350,7 +350,7 @@ Combining Values: Interpolation, Addition, and Accumulation</h2>
 	Some procedures, for example
 	<a href="https://www.w3.org/TR/css-transitions/">transitions</a>
 	and <a href="https://www.w3.org/TR/css-animations/">animations</a>,
-	combine two CSS property values.
+	<dfn export>combine</dfn> two CSS property values.
 	The following combining operations--
 	on the two <a>computed values</a> <var>V<sub>a</sub></var> and <var>V<sub>B</sub></var>
 	yielding the <a>computed value</a> <var>V<sub>result</sub></var>--