[css-overflow-5] Define an initial focus order for the scrolling pseu…

…do-elements. #10912

css-overflow-5/Overview.bs

@@ -433,6 +433,39 @@ Issue: The UA stylesheet needs to specify that ''::scroll-button()''s
 are styled identically to the <{button}> element.
 
 
+<h3 id=focus-order>
+Focus Navigation Order</h3>
+
+The above features generate several focusable pseudo-elements.
+While these pseudo-elements have a defined position in the element tree,
+this isn't an optimal position for focus navigation
+(aka "tab order")
+for these controls.
+
+Instead,
+focus navigation between a [=scroll container=]
+and the various pseudo-elements defined in this section
+goes in the following order:
+
+1. The ''::scroll-marker-group'' pseudo-elements of the [=scroll container=],
+	if it is set to ''scroll-marker-group: before''.
+
+	Note: The individual ''::scroll-marker'' pseudo-elements
+	generated by the [=scroll containers=] descendants
+	are reparented underneath this ''::scroll-marker-group'',
+	and navigated together as a "focus group".
+
+2. The ''::scroll-button()'' pseudo-elements,
+	in the order they're defined as existing in.
+
+3. The [=scroll container=] itself,
+	and its contents,
+	in the normal focus order they would be in.
+
+4. The ''::scroll-marker-group'' pseudo-elements of the [=scroll container=],
+	if it is set to ''scroll-marker-group: after''.
+
+
 <h2 id="fragmentation" class=no-num>
 Appendix A: Redirection of Overflow</h2>