[css-values-5] Add transform-mix()

Author: tabatkins

css-values-5/Overview.bs

@@ -376,6 +376,32 @@ Interpolated Image Values: the ''cross-fade()'' notation</h3>
 	as the <<percentage>> of the first <<color>> argument in the second variant.
 	See [[css-images-4#cross-fade-function]] for the normative definition of the second variant.
 
+
+<h3 id="transform-mix">
+Interpolated Transform Values: the ''transform-mix()'' notation</h3>
+
+	The <dfn>transform-mix()</dfn> [=mix notation=]
+	represents an interpolated <<transform-list>>,
+	with the following syntactic form:
+
+	<pre class=prod>
+		<dfn><<transform-mix()>></dfn> = transform-mix( <<progress>>, <<transform-list>>, <<transform-list>> )
+	</pre>
+
+	The [=used value=] of a valid ''transform-mix()'' is
+	the result of interpolating these two values
+	to the progress given by <<progress>>.
+	If the <<progress>> value can be computed to a <<percentage>>,
+	and the <<transform-list>>s can be interpolated
+	without used-value-time information,
+	then the [=computed value=] is likewise
+	the result of interpolating the two [=computed values=]
+	to that <<progress>> value;
+	it is otherwise the ''transform-mix()'' notation itself
+	with its arguments each computed according to their type.
+
+	''transform-mix()'' is, itself, a <<transform-function>>.
+
 <h3 id="mix">
 Interpolated Property Values: the ''mix()'' notation</h3>