[css-overflow-3] Define that UAs must start with the assumption of no scrollbars when calculating sizes w3c#3348 (per https://www.w3.org/TR/2007/WD-css3-box-20070809/#collapse-scroll)

Author: fantasai

css-overflow-3/Overview.bs

@@ -493,6 +493,9 @@ Scrollbars and Layout</h3>
 	When the box is sized to fit its contents,
 	this reserved space is added to the size of its contents.
 	It is otherwise subtracted from space alotted to the [=content area=].
+	To the extent that the presence of scrollbars can affect sizing,
+	UAs must start with the assumption that no scrollbars are needed,
+	and recalculate sizes if it turns out they are.
 
 	Issue: import examples from [[CSS3-BOX]].