[css-grid-1] Require grid-template-areas to define at least one grid …

…cell. #5110

css-grid-1/Overview.bs

@@ -2108,7 +2108,9 @@ Named Areas: the 'grid-template-areas' property</h3>
 					and makes the declaration invalid.
 			</ul>
 
-			All strings must have the same number of columns,
+			All strings must define the same number of cell tokens
+			([=named cell tokens=] and/or [=null cell tokens=]),
+			and at least one cell token,
 			or else the declaration is invalid.
 			If a <a>named grid area</a> spans multiple <a>grid cells</a>,
 			but those cells do not form a single filled-in rectangle,
@@ -5031,6 +5033,10 @@ Minor Changes</h4>
 				is <a lt="generate baselines">generated</a> from
 				the shared <a>alignment baseline</a> of those <a>grid items</a>.
 			</blockquote>
+
+		<li id="change-2017-at-least-one-cell">
+			Required 'grid-template-areas' to define at least one cell in the grid;
+			''grid-template-areas: "" "";'' is invalid.
 	</ul>
 
 <h4 id="clarify-2017">