[css-grid] Clarifications to subgrid

Author: fantasai

css-grid/Overview.html

@@ -1092,21 +1092,31 @@ <h4 data-level=5.1.4 id=subgrids><span class=secno>5.1.4 </span><span class=cont
 		<li>
 			The <a href=#subgrid>subgrid</a>'s own grid items participate in the sizing of its parent grid and are aligned to it.
 			In this process, the sum of the item's margin, padding, and borders are applied as an extra layer of margin to the items at those edges.
+			However, any overflow tracks
+			(i.e. those outside the <a href=#explicit-grid>explicit grid</a> when the <a href=#subgrid>subgrid</a> has a <a href=#definite>definite grid span</a>)
+			do not correspond to any tracks in the parent grid;
+			they effectively extend in a third dimension.
+			(For example, if a parent grid has adjacent tracks <var>A</var>, <var>B</var>, and <var>C</var>,
+			and a <span class=css data-link-type=maybe>span 1</span> subgrid with an extra <a href=#implicit-grid-tracks>implicit grid track</a> is placed in track <var>B</var>,
+			the items in that <a href=#implicit-grid-tracks>implicit grid track</a> are not considered part of track <var>B</var>.)
 
 		<li>
 			The <a href=#subgrid>subgrid</a> is always stretched;
 			the <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-align-3/#align-self title=align-self>align-self</a>/<a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-align-3/#justify-self title=justify-self>justify-self</a> properties on it are ignored.
 			Any specified width/height constraints are also ignored.
 
 		<li>
-			The <a href=#subgrid>subgrid</a>’s <a href=#explicit-grid>explicit grid</a> is always aligned with the corresponding section of the parent <a href=#grid>grid</a>;
+			Layoutwise, the <a href=#subgrid>subgrid</a>’s <a href=#explicit-grid>explicit grid</a> is always aligned with the corresponding section of the parent <a href=#grid>grid</a>;
 			the <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-align-3/#align-content title=align-content>align-content</a>/<a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-align-3/#justify-content title=justify-content>justify-content</a> properties on it are ignored.
+			However, <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css-overflow-3/#overflow title=overflow>overflow</a> does apply,
+			so the contents of the subgrid can be scrolled aside.
+			(Scrolling does not affect layout.)
 
 		<li>
 			Explicit named lines can also be specified together with the <a data-link-type=value href=#valuedef-subgrid>subgrid</a> keyword;
 			these names apply to the corresponding lines taken from the parent <a href=#grid>grid</a>.
 			If the <a href=#subgrid>subgrid</a> has an explicit <a href=#grid-span>grid span</a>,
-			any names for lines beyond the span are ignored.
+			any names specified for lines beyond the span are ignored.
 			If the <a href=#subgrid>subgrid</a> has an explicit <a href=#grid-position>grid position</a> as well as an explicit <a href=#grid-span>grid span</a>,
 			it also automatically receives the names of the lines that it covers in its parent <a href=#grid>grid</a>.
 	</ul>

css-grid/Overview.src.html

@@ -1024,21 +1024,31 @@ <h4 id="subgrids">
 		<li>
 			The <a>subgrid</a>'s own grid items participate in the sizing of its parent grid and are aligned to it.
 			In this process, the sum of the item's margin, padding, and borders are applied as an extra layer of margin to the items at those edges.
+			However, any overflow tracks
+			(i.e. those outside the <i>explicit grid</i> when the <i>subgrid</i> has a <i>definite grid span</i>)
+			do not correspond to any tracks in the parent grid;
+			they effectively extend in a third dimension.
+			(For example, if a parent grid has adjacent tracks <var>A</var>, <var>B</var>, and <var>C</var>,
+			and a ''span 1'' subgrid with an extra <i>implicit grid track</i> is placed in track <var>B</var>,
+			the items in that <i>implicit grid track</i> are not considered part of track <var>B</var>.)
 
 		<li>
 			The <a>subgrid</a> is always stretched;
 			the 'align-self'/'justify-self' properties on it are ignored.
 			Any specified width/height constraints are also ignored.
 
 		<li>
-			The <a>subgrid</a>’s <a>explicit grid</a> is always aligned with the corresponding section of the parent <a>grid</a>;
+			Layoutwise, the <a>subgrid</a>’s <a>explicit grid</a> is always aligned with the corresponding section of the parent <a>grid</a>;
 			the 'align-content'/'justify-content' properties on it are ignored.
+			However, 'overflow' does apply,
+			so the contents of the subgrid can be scrolled aside.
+			(Scrolling does not affect layout.)
 
 		<li>
 			Explicit named lines can also be specified together with the <a value>subgrid</a> keyword;
 			these names apply to the corresponding lines taken from the parent <a>grid</a>.
 			If the <a>subgrid</a> has an explicit <a>grid span</a>,
-			any names for lines beyond the span are ignored.
+			any names specified for lines beyond the span are ignored.
 			If the <a>subgrid</a> has an explicit <a>grid position</a> as well as an explicit <a>grid span</a>,
 			it also automatically receives the names of the lines that it covers in its parent <a>grid</a>.
 	</ul>