[css-grid-2] Define the explicit grid of a subgrid and specify how line numbers work more clearly. #2564

Author: fantasai

css-grid-2/Overview.bs

@@ -145,12 +145,22 @@ Characteristics of a Subgrid Item</h3>
 			rather than by 'grid-template-rows'/'grid-template-columns'.
 			The track sizes are governed by the parent grid.
 
+			The grid lines thus shared between the subgrid and its parent
+			form the subgrid’s <a>explicit grid</a>.
+
 		<li id="subgrid-indexing">
 			The <a>grid-placement properties</a> of the <a>subgrid</a>’s <a>grid items</a>
-			are scoped to the lines covered by the subgrid.
-			E.g., numeric indices count starting from the first line of the subgrid
+			and the line numbers they use
+			are scoped to the lines covered by the subgrid,
+			exactly consistent with the lines outside the subgrid
+			being excluded from its <a>explicit grid</a>.
+			E.g. numeric indices count starting from the first line of the subgrid
 			rather than the first line of the parent grid.
 
+			Line numbering and placement rules
+			obey the subgrid’s own <a>writing mode</a>,
+			just as they would for a nested independent grid.
+
 		<li id="subgrid-line-name-inheritance">
 			Since <a>subgrids</a> can be placed before their contents are placed,
 			the subgridded lines