[css-values-5] Provide a grammar for <input-position>. #6245

Author: tabatkins

css-values-5/Overview.bs

@@ -1126,6 +1126,15 @@ Defining the Interpolation Map</h4>
 			if two <<input-position>>s are specified,
 			it is treated the same as two stops with the same <<output-value>>.
 
+			<pre class=prod>
+				<input-position> = <percentage> | <number> | <dimension>
+			</pre>
+
+			Note: <<output-value>> is not given a grammar here,
+			as the specific functions specify what their outputs are.
+			<<input-position>>, however, is linked to <<progress-source>>,
+			see [[#interpolation-validity]].
+
 		<dt><dfn value><<easing-function>></dfn>
 		<dd>
 			When appearing in the first argument,