[css-display-4] Inline background logic from issue into a note w3c#7387

Author: fantasai

css-display-4/Overview.bs

@@ -978,13 +978,45 @@ Display Order</h2>
 	Advisement: Author-facing documentation such as tutorials and references
 	should adequately address the accessibility impacts
 	of 'order', 'layout-order', and 'reading-order'
-	and discourage their misuse.
-
-	ISSUE(7387): See this
-	<a href="https://github.com/w3c/csswg-drafts/issues/7387#issuecomment-1217193918">
-	summary of how this all fits together and why</a>.
-
-	ISSUE: DOM needs a convenient reordering function
+	and discourage their misuse by authors and authoring tools.
+
+	<details class="note">
+		<summary>Design Considerations and Background</summary>
+
+		Some of the considerations that went into the design
+		of 'order', 'reading-order', and 'layout-order' are:
+
+		* There are clear use cases for disconnecting
+			the [=reading and navigation order=] from the box layout order,
+			the most fundamental of which is to make sure
+			the [=reading and navigation order=] matches the <strong>visual perception order</strong>
+			when it is not the same as the box layout order.
+			(Visual perception is non-linear, and is influenced by things like
+				the size, color contrast, and spacing of a visual element,
+				not just its spatial coordinates on the page.)
+		* It is a <a href="https://www.w3.org/TR/webarch/#pci">core principle of Web platform architecture</a>,
+			in order to allow content to be accessible to the widest possible audience
+			across devices that exist now and in the future,
+			for the underlying document to be sensical independent of CSS.
+			Therefore the underlying document order
+			should represent a logical ordering of its elements
+			regardless of its visual presentation.
+		* For components of a page that do not have a strong inherent order,
+			a document can have multiple visual presentations with different layouts,
+			all conveying the same semantic information.
+			It should be possible for all of these presentations to have good accessibility.
+		* Linear navigation, focus sequencing order, and screen-reader order
+			should always match, because there are users who use them together.
+		* Each component or hierarchical level of a page
+			can have different requirements for reordering,
+			so CSS reordering controls should not lend themselves
+			too easily to blanket use
+			(like 'box-sizing')
+			rather than tailored use
+			(like <a href="https://www.w3.org/TR/css-logical/">flow-relative vs. physical properties and values</a>).
+	</details>
+
+	ISSUE(7387): DOM needs a convenient reordering function
 	so that authors (even authors who don't usually write JS)
 	can easily perform source order reordering when necessary
 	instead of misusing 'order'.