[css-overflow-5] Default style of ::scroll-marker and ::scroll-button(*) is like link and button respectively (#12030)

Per the resolution in #10935, the default style of ::scroll-marker pseudo-elements is like links,
and ::scroll-button(*) pseudo-elements is like buttons including switching based on the appearance property.

Author: flackr

css-overflow-5/Overview.bs

@@ -292,6 +292,8 @@ The 'scroll-marker-group' property</h4>
 	on its nearest ancestor [=scroll container=].
 	These boxes are added in the [=tree order=] of their <a>originating element</a>.
 
+	These pseudo-elements are [=fully styleable pseudo-elements=] and have a default style like the [[html#the-a-element|HTML &lt;a> element]].
+
 	These pseudo-elements have an indicated [=scroll target=] of their <a>originating element</a>.
 	They behave as an element with a <a href="https://html.spec.whatwg.org/multipage/interaction.html#attr-tabindex"><code>tabindex</code></a> of "-1",
 	making them focusable within their '::scroll-marker-group' either by arrow key navigation within the group,
@@ -502,6 +504,7 @@ 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 styleable pseudo-elements=]: there is no restriction on what properties apply to them.
+They are styled like buttons, switching between the <a>native appearance</a> and <a>base appearance</a> based on the 'appearance' property value.
 
 The '':disabled'' pseudo-class can apply to ''::scroll-button()''.
 It matches a given button