[css-grid] Allow grid-auto-rows/cols to take multiple track listings per WG resolution

Author: fantasai

css-grid/Overview.bs

@@ -1927,7 +1927,7 @@ Implicit Track Sizing: the 'grid-auto-rows' and 'grid-auto-columns' properties</
 
 	<pre class='propdef'>
 	Name: grid-auto-columns, grid-auto-rows
-	Value: <<track-size>>
+	Value: <<track-size>>+
 	Initial: auto
 	Applies to: <a>grid containers</a>
 	Inherited: no
@@ -1943,6 +1943,13 @@ Implicit Track Sizing: the 'grid-auto-rows' and 'grid-auto-columns' properties</
 	or by the <a>auto-placement algorithm</a> creating additional rows or columns.
 	The 'grid-auto-columns' and 'grid-auto-rows' properties specify the size of such implicitly-created tracks.
 
+	If multiple track sizes are given, the pattern is repeated as necessary
+	to find the size of the implicit tracks.
+	The first <a>implicit grid track</a> before the <a>explicit grid</a>
+	receives the first specified size, and so on forwards;
+	and the last <a>implicit grid track</a> before the <a>explicit grid</a>
+	receives the last specified size, and so on backwards.
+
 	<div class="example">
 		<pre>
 			&lt;style>