[css-overflow-5] Specify that ::scroll-marker-group and ::scroll-butt…

…on() generate as siblings of the originating element. #11125

css-overflow-5/Overview.bs

@@ -228,10 +228,13 @@ The 'scroll-marker-group' property</h4>
 
 <h4 id="scroll-marker-group-pseudo">The ''::scroll-marker-group'' pseudo-element</h4>
 
-	The <dfn selector>::scroll-marker-group</dfn> [=fully stylable pseudo-element=]
+	The <dfn selector>::scroll-marker-group</dfn> [=fully styleable pseudo-element=]
 	is generated by a [=scroll container=] element
-	having a computed 'scroll-marker-group' property that is not 'none',
-	adjacent to the [=scroll container=].
+	having a computed 'scroll-marker-group' property that is not 'none'.
+
+	The ''::scroll-marker-group'' generates a box as a <em>sibling</em> of its [=originating element=],
+	either immediately preceding (if ''scroll-marker-group: before'')
+	or immediately following (if ''scroll-marker-group: after'').
 
 	The following additions are recommended for the default UA stylesheet
 	to ensure that the generation of scroll marker pseudo-elements does not invalidate the layout of the site:
@@ -364,13 +367,11 @@ Scroll Buttons</h3>
 The <dfn>::scroll-button( <<scroll-button-direction>> )</dfn> pseudo-elements
 are generated on [=scroll containers=]
 when their computed 'content' value is not ''content/none''.
-They generate boxes as if they were immediate children
+They generate boxes as if they were immediately preceding <em>siblings</em>
 of their [=originating element=],
 with content as specified by 'content'.
 They exist after their [=originating element's=] ''::scroll-marker-group'' pseudo-element
-(when used with ''scroll-marker-group: before'')
-and before its ''::before'' pseudo-element,
-if those exist.
+(when used with ''scroll-marker-group: before'').
 
 Four distinct ''::scroll-button()'' pseudo-elements can exist on a [=scroll container=],
 each associated with a [=logical=] direction,
@@ -445,7 +446,7 @@ to make it easier to style several buttons the same way?
 In particular, <css>all</css> is probably useful,
 but maybe also <css>horizontal</css>/<css>vertical</css>/<css>block</css>/<css>inline</css>.
 
-The ''::scroll-button()''s are [=fully stylable pseudo-elements=]: there is no restriction on what properties apply to them.
+The ''::scroll-button()''s are [=fully styleable pseudo-elements=]: there is no restriction on what properties apply to them.
 
 The '':disabled'' pseudo-class can apply to ''::scroll-button()''.
 It matches a given button