[css-grid-2] Specify the computed and resolved values of a subgrid's grid-template-*. #4362

Author: tabatkins

css-grid-2/Overview.bs

@@ -101,6 +101,7 @@ Establishing a Subgrid</h3>
 	<pre class="propdef partial">
 	Name: grid-template-rows, grid-template-columns
 	New values: subgrid <<line-name-list>>?
+	New computed values: The ''subgrid'' keyword followed by a <<line-name-list>>.
 	</pre>
 
 	<dl dfn-for="grid-template-rows, grid-template-columns" dfn-type=value>
@@ -464,6 +465,62 @@ Characteristics of a Subgrid Item</h3>
 			(Note: the act of scrolling does not affect layout.)
 	</ul>
 
+<!--
+████████  ████████  ██████   ███████  ██       ██     ██ ████████ ████████
+██     ██ ██       ██    ██ ██     ██ ██       ██     ██ ██       ██     ██
+██     ██ ██       ██       ██     ██ ██       ██     ██ ██       ██     ██
+████████  ██████    ██████  ██     ██ ██       ██     ██ ██████   ██     ██
+██   ██   ██             ██ ██     ██ ██        ██   ██  ██       ██     ██
+██    ██  ██       ██    ██ ██     ██ ██         ██ ██   ██       ██     ██
+██     ██ ████████  ██████   ███████  ████████    ███    ████████ ████████
+-->
+
+<h3 id='resolved-track-list'>
+Resolved Value of a Track Listing</h3>
+
+	When an element generates a [=grid container=] box that is a [=subgrid=],
+	the [=resolved value=]
+	of the 'grid-template-rows' and 'grid-template-columns' properties is the <a>used value</a>,
+	serialized as the ''subgrid'' keyword
+	followed by a list representing each of its lines
+	as a [=line name set=] of all the line's names,
+	whether the names were explicitly created for the subgrid
+	or adopted from the parent grid,
+	without using the ''repeat()'' notation.
+
+	<div class="example">
+		For example,
+		when applied to a [=subgrid=] with ''grid-column: span 4'',
+		each of the following 'grid-template-columns' [=specified values=]
+		becomes the corresponding [=resolved values=]:
+
+		<pre class="lang-css">
+			specified: subgrid [a] repeat(auto-fill, [b]) [c]
+			resolved:  subgrid [a] [b] [b] [b] [c]
+		</pre>
+
+		<pre class=lang-css>
+			specified: subgrid [a] [a] [a] [a] repeat(auto-fill, [b]) [c] [c]
+			resolved:  subgrid [a] [a] [a] [a] [c]
+		</pre>
+
+		<pre class="lang-css">
+			specified: subgrid [] [a]
+			resolved:  subgrid [] [a] [] [] []
+		</pre>
+
+		<pre class="lang-css">
+			specified: subgrid [a] [b] [c] [d] [e] [f]
+			resolved:  subgrid [a] [b] [c] [d] [e]
+		</pre>
+	</div>
+
+	Note: This violates the general
+	"shortest equivalent serialization" principle
+	by serializing empty trailing [=line name sets=],
+	as the trailing [=line name sets=] provide potentially-useful information
+	about how many tracks the subgrid is spanning.
+
 <h3 id="subgrid-sizing">
 Subgrid Sizing Algorithm</h3>