Add back the 'at least one million tracks' requirement, which we resolved on back in Jan 2015 and forgot to add back in. Related to #2261.

Author: tabatkins

css-grid-1/Overview.bs

@@ -926,7 +926,8 @@ Clamping Overly Large Grids</h3>
 
 	Since memory is limited,
 	UAs may clamp the possible size of the <a>grid</a>
-	to be within a UA-defined limit,
+	to be within a UA-defined limit
+	(which should be at least one million tracks),
 	dropping all lines outside that limit.
 	If a grid item is placed outside this limit,
 	its grid area must be <a>clamped</a> to within this limited grid.
@@ -4256,6 +4257,20 @@ Major Changes</h4>
 			to be resolved against the block dimension;
 			they must be resolved against the inline dimension, as for blocks.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/2085">Issue 2085</a>)
+
+		<li id="change-2017-million-tracks">
+			Specify a minimum number of tracks that a UA should support
+			(absent memory pressure or similar).
+			(<a href="https://github.com/w3c/csswg-drafts/issues/2261">Issue 2261</a>)
+			<blockquote>
+				<p>Since memory is limited,
+				UAs may clamp the possible size of the <a>grid</a>
+				to be within a UA-defined limit
+				<ins>(which should be at least one million tracks)</ins>,
+				dropping all lines outside that limit.
+				If a grid item is placed outside this limit,
+				its grid area must be <a>clamped</a> to within this limited grid.
+			</blockquote>
 	</ul>
 
 <h4 id="clarify-2017">