[css-values-5] Specify the generic computed/used behavior for *-mix(), copying from *-interpolate(). #6245

Author: tabatkins

css-values-5/Overview.bs

@@ -912,6 +912,18 @@ Normalizing Mix Percentages</h3>
 		mixed into the final result.
 	</div>
 
+<h3 id=mix-resolution>
+Mix Resolution</h3>
+
+	The [=used value=] of a valid [=mix notation=] is
+	the weighted average of its arguments,
+	as specified by the specific notation.
+
+	The [=computed value=] is the [=used value=] if it is possible to calculate.
+	Otherwise,
+	it is the [=mix notation=] itself,
+	with its arguments computed individually.
+
 <h3 id="calc-mix">
 Weighted Average of Numeric and Dimensional Values: the ''calc-mix()'' notation</h3>