[css-grid-1] Define interpolation of repeat() functions. w3c#3503

Author: fantasai

css-grid-1/Overview.bs

@@ -1374,7 +1374,7 @@ Explicit Track Sizing: the 'grid-template-rows' and 'grid-template-columns' prop
 	Inherited: no
 	Percentages: refer to corresponding dimension of the content area
 	Computed value: the keyword ''grid-template-columns/none'' or a [=computed track list=]
-	Animation type: if the list lengths match, by computed value type per item in the [=computed track list=]; discrete otherwise
+	Animation type: if the list lengths match, by computed value type per item in the [=computed track list=] (see [[#computed-tracks]] and [[#repeat-interpolation]]); discrete otherwise
 	</pre>
 
 	These properties specify,
@@ -1716,6 +1716,18 @@ Repeat-to-fill: ''auto-fill'' and ''auto-fit'' repetitions</h5>
 	to avoid division by zero.
 	It is suggested that this floor be ''1px''.
 
+<h5 id="repeat-interpolation">
+Interpolation of ''repeat()''</h5>
+
+	If two ''repeat()'' notations that
+	have the same first argument (repetition count)
+	and the same number of tracks in their second argument (the track listing),
+	they are interpolated by
+	interpolating each component of their <a>computed track lists</a>
+	<a>by computed value</a>
+	(just like interpolating a top-level track list).
+	They otherwise interpolate as <a>discrete</a>.
+
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