[css-grid-3] Expand the grid-independent section for track definitions

Author: tabatkins

css-grid-3/Overview.bs

@@ -426,14 +426,58 @@ Masonry Track Specification</h3>
 <span class="option masonry">Grid-independent Option:</span> Declaring Masonry Track Templates: the 'masonry-template-*' properties</h3>
 
 	Tracks in the [=grid axis=] of a [=masonry container=]
-	are established with the
-	<dfn property>masonry-template-tracks</dfn>
-	and <dfn property>masonry-template-areas</dfn> properties,
-	whose syntax and interpretation are analogous to the
-	'grid-template-columns' and 'grid-template-areas' properties.
+	are established with 'masonry-template-tracks',
+	'masonry-template-areas',
+	and 'masonry-auto-tracks'.
+
+	<pre class=propdef>
+	Name: masonry-template-tracks
+	Value: <<'grid-template-columns'>>
+	Initial: repeat(auto-areas, auto)
+	Applies to: [=masonry containers=]
+	Inherited: no
+	Percentages: refer to correspodning dimension of the content area
+	Computed value: by computed value type per item in the [=computed track list=]
+	Animation type: if list lengths match, by [=computed value=] type; otherwise, discrete
+	</pre>
+
+	'masonry-template-tracks' has the same syntax and interpretation as 'grid-template-columns',
+	but a different initial value.
+
+	(Instead of defaulting to no tracks, as in Grid Layout,
+	it defaults to matching 'masonry-template-areas', if possible,
+	or otherwise filling the [=masonry container=]
+	with as many ''grid-template-columns/auto'' tracks as possible.)
+
+	<pre class=propdef>
+	Name: masonry-template-areas
+	Value: none | <<string>>
+	Initial: none
+	Applies to: [=masonry containers=]
+	Inherited: no
+	Percentages: n/a
+	Computed Value: the keyword ''masonry-template-values/none'' or a string
+	Canonical Order: per grammar
+	Animation type: discrete
+	</pre>
+
+	'masonry-template-areas' has the same syntax and interpretation as 'grid-template-areas',
+	except it only takes a single string defining one "row" of area names.
+
+	<pre class=propdef>
+	Name: masonry-auto-tracks
+	Value: <<'grid-auto-columns'>>
+	Initial: auto
+	Applies to: grid containers
+	Inherited: no
+	Percentages: refer to correspodning dimension of the content area
+	Computed value: a [=computed track list=]
+	Canonical order: per grammar
+	Animation type:	if the list lengths match, by computed value type per item; discrete otherwise
+	</pre>
+
+	'masonry-auto-tracks' has the same syntax and interpretation as 'grid-auto-columns'.
 
-	ISSUE: Should there be a <css>masonry-auto-tracks</css> property,
-	analogous to 'grid-auto-columns'?
 </section>
 
 <h3 id="subgrids">