[css-overflow-3] Clarify Overflow Value Propagation

The wording in CSS2.1 was more precise than the one used here,
so bring it back (with editorial modernization).

Closes #1905

Author: frivoal

css-overflow-3/Overview.bs

@@ -375,13 +375,14 @@ Overflow Value Propagation</h3>
 
 	UAs must apply the 'overflow' property
 	set on the root element to the viewport.
-	For HTML UAs,
-	if this would result in the viewport having ''visible'' overflow,
-	the UA must instead apply the 'overflow' property
-	set on the <{body}> element to the viewport.
-	In either case,
-	the used value of 'overflow' for the element from which the value is propagated
-	must evaluate to ''visible''.
+	When the root element is an [[!HTML]] <{html}> element
+	(including <a href="https://html.spec.whatwg.org/multipage/introduction.html#html-vs-xhtml">XML syntax for HTML</a>),
+	and that element has a <{body}> element as a child,
+	user agents must instead apply the 'overflow' property
+	from the first such child element to the viewport,
+	if the value on the root element is ''overflow/visible''.
+	The ''overflow/visible'' value when used for the viewport must be interpreted as ''overflow/auto''.
+	The element from which the value is propagated must have a used value for 'overflow' of ''overflow/visible''.
 
 	If this application would result in the viewport having ''visible'' overflow,
 	the UA must instead treat the viewport as having ''overflow/auto'' overflow.