[css-flexbox] Define main size/cross size for flex containers. (Curre…

…nt definition only applies to flex items.) Fixes #981.

css-flexbox/Overview.bs

@@ -440,11 +440,11 @@ Flex Layout Box Model and Terminology</h2>
 		<dt class='size'>main size
 		<dt class='size'>main size property
 		<dd>
-			A <a>flex item</a>’s width or height,
+			The width or height of a <a>flex container</a> or <a>flex item</a>,
 			whichever is in the <a>main dimension</a>,
-			is the item's <dfn lt="main size|main-size">main size</dfn>.
-			The <a>flex item</a>’s <dfn lt="main size property|max main size property|min main size property">main size property</dfn> is
-			either the 'width' or 'height' property,
+			is that box’s <dfn lt="main size|main-size">main size</dfn>.
+			Its <dfn lt="main size property|max main size property|min main size property">main size property</dfn> is
+			thus either its 'width' or 'height' property,
 			whichever is in the <a>main dimension</a>.
 	</dl>
 
@@ -465,11 +465,12 @@ Flex Layout Box Model and Terminology</h2>
 		<dt class='size'>cross size
 		<dt class='size'>cross size property
 		<dd>
-			The width or height of a <a>flex item</a>,
+			The width or height of a <a>flex container</a> or <a>flex item</a>,
 			whichever is in the <a>cross dimension</a>,
-			is the item's <dfn lt="cross size | cross-size">cross size</dfn>.
-			The <dfn lt="cross size property|max cross size property|min cross size property">cross size property</dfn> is
-			whichever of 'width' or 'height' that is in the <a>cross dimension</a>.
+			is that box’s <dfn lt="cross size | cross-size">cross size</dfn>.
+			Its <dfn lt="cross size property|max cross size property|min cross size property">cross size property</dfn> is
+			thus either its 'width' or 'height' property,
+			whichever is in the <a>cross dimension</a>.
 	</dl>
 
 	Additional sizing terminology used in this specification
@@ -3460,6 +3461,9 @@ Substantive Changes and Bugfixes</h4>
 <h4 id="change-201605-clarify">
 Clarifications</h4>
 	<ul>
+		<li>Made sure that <a>main size</a> and <a>cross size</a> are defined
+			for <a>flex containers</a> as well as for <a>flex items</a>.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/981">Issue 981</a>)
 	</ul>
 
 <h3 id="changes-20160301">