[css-display-4] Define reading flow container

Author: rachelandrew

css-display-4/Overview.bs

@@ -1151,6 +1151,14 @@ Reordering and Accessibility</h3>
 <h2 id='reading-flow'>
 Reading Order: the 'reading-flow' property</h2>
 
+A <dfn export>reading flow container</dfn> is a flex or grid container
+with a valid 'reading-flow' value other than 'normal'.
+
+The rendering-defined sibling reading flow of a <a>reading flow container</a>
+is an ordered list of in-flow children of the container.
+All children must render to an element and are considered siblings in the reading flow container.
+The order is determined by the 'reading-flow' property.
+
 ISSUE(9922): Should this property also apply to tables?
 
 ISSUE(9230): Define how reading-flow interacts with focusable display: contents elements.
@@ -1199,6 +1207,7 @@ ISSUE(9230): Define how reading-flow interacts with focusable display: contents
 			taking the writing mode into account.
 		<dt><dfn>grid-order</dfn>
 		<dd>
+			Only takes effect on grid containers.
 			Follows the [=order-modified document order=].
 			Therefore, as 'normal' unless the 'order'
 			property has been used to change the order of items.
@@ -1304,16 +1313,6 @@ ISSUE(9230): Define how reading-flow interacts with focusable display: contents
 	should be encoded in the source order
 	so that the document is sensical without CSS.
 
-	If the host language defines features for explicitly controlling
-	the reading or navigation order
-	(such as <a href="https://html.spec.whatwg.org/multipage/interaction.html#attr-tabindex"><code>tabindex</code></a> [[HTML]]),
-	these take precedence over 'reading-flow':
-	the modified document order created by 'reading-flow'
-	essentially replaces the underlying source order
-	for the purpose of such features.
-
-	ISSUE: Is this the most appropriate interaction with with <code>tabindex</code>?
-
 	<details class="note">
 		<summary>Design Considerations and Background</summary>