[css-flexbox] Add a11y conformance criteria for authoring tools, per WG resolution.

Author: fantasai

css-flexbox/Overview.bs

@@ -1235,8 +1235,8 @@ Reordering and Accessibility</h4>
 	(such as cycling through links, see e.g. <a href="http://www.w3.org/TR/html5/editing.html#sequential-focus-navigation-and-the-tabindex-attribute"><code>tabindex</code></a> [[HTML5]]).
 
 	Advisement:
-	Authors <em>must</em> use 'order' only for visual, not logical, reordering of content;
-	style sheets that use 'order' to perform logical reordering are non-conforming.
+	Authors <em>must</em> use 'order' only for visual, not logical, reordering of content.
+	Style sheets that use 'order' to perform logical reordering are non-conforming.
 
 	Note: This is so that non-visual media and non-CSS UAs,
 	which typically present content linearly,
@@ -1306,6 +1306,33 @@ Reordering and Accessibility</h4>
 		<small>(Further use of multi-line flex containers to achieve even more intelligent wrapping left as an exercise for the reader.)</small>
 	</div>
 
+	In order to preserve the author's intended ordering in all presentation modes,
+	authoring tools&mdash;including WYSIWYG editors as well as Web-based authoring aids--
+	must reorder the underlying document source
+	and not use 'order' to perform reordering
+	unless the author has explicitly indicated that the underlying
+	document order (which determines speech and navigation order) should be
+	<em>out-of-sync</em> with the visual order.
+
+	<div class="example">
+		For example, a tool might offer both drag-and-drop reordering of flex items
+		as well as handling of media queries for alternate layouts per screen size range.
+
+		Since most of the time, reordering should affect all screen ranges
+		as well as navigation and speech order,
+		the tool would perform drag-and-drop reordering at the DOM layer.
+		In some cases, however, the author may want different visual orderings per screen size.
+		The tool could offer this functionality by using 'order' together with media queries,
+		but also tie the smallest screen size's ordering to the underlying DOM order
+		(since this is most likely to be a logical linear presentation order)
+		while using 'order' to determine the visual presentation order in other size ranges.
+
+		This tool would be conformant, whereas a tool that only ever used 'order'
+		to handle drag-and-drop reordering
+		(however convenient it might be to implement it that way)
+		would be non-conformant.
+	</div>
+
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@@ -3302,6 +3329,12 @@ Substantive Changes and Bugfixes</h4>
 			since that was a hacky way of solving an intrinsic size problem,
 			and isn't needed (and gives bad results)
 			given a correct implementation of [[#intrinsic-sizes]].
+		<li id="change-2015-intrinsic-sizing">
+			Correct instrinsic sizing rules to handle inflexible items.
+		<li id="change-2015-a11y-tools">
+			Add explicit conformance criteria on authoring tools
+			to keep presentation and DOM order in sync
+			unless author explicitly indicates a desire to make them out-of-sync.
 	</ul>
 
 <h4 id="change-201505-clarify">