[css-grid-2] Define subgrids with an automatic span to take their span from the number of explicit tracks. #2565

Author: fantasai

css-grid-2/Overview.bs

@@ -147,13 +147,36 @@ Characteristics of a Subgrid Item</h3>
 
 	<ul style="list-style-type: lower-alpha">
 		<li id="subgrid-tracks">
-			The number of explicit tracks in each subgridded dimension
-			is given by its <a>grid span</a> in that dimension,
-			rather than by 'grid-template-rows'/'grid-template-columns'.
-			The track sizes are governed by the parent grid.
-
+			Placing the <a>subgrid</a>
+			creates a correspondence between its subgridded tracks
+			and those that it spans in its parent grid.
 			The grid lines thus shared between the subgrid and its parent
-			form the subgrid’s <a>explicit grid</a>.
+			form the subgrid’s <a>explicit grid</a>,
+			and its track sizes are governed by the parent grid.
+
+		<li id="subgrid-span">
+			The number of explicit tracks in the <a>subgrid</a>
+			in a subgridded dimension
+			always corresponds to the number of <a>grid tracks</a>
+			that it spans in its parent <a>grid</a>:
+
+			<ul>
+				<li>
+					If the <a>subgrid</a>’s <a>grid span</a> in the subgridded dimension is definite,
+					then the number of explicit tracks in each subgridded dimension
+					is taken from its used <a>grid span</a> in that dimension
+					(regardless of its 'grid-template-*' properties).
+
+				<li>
+					If it has an <a>indefinite grid span</a>
+					(i.e. either the '-start' or '-end' value
+					of its <a>grid-placement properties</a> in the subgridded axis
+					is ''grid-row-start/auto'')
+					then its used <a>grid span</a> is taken
+					from the number of explicit tracks specified for that axis
+					by its 'grid-template-*' properties,
+					floored at one.
+			</ul>
 
 		<li id="subgrid-indexing">
 			The <a>grid-placement properties</a> of the <a>subgrid</a>’s <a>grid items</a>