[css-overflow-3] Wrt applying 'overflow' to replaced elements add UA stylesheet expectations and note about change from 2.1 w3c#7058 w3c#7144

Author: fantasai

css-overflow-3/Overview.bs

@@ -478,6 +478,16 @@ Scrolling and Clipping Overflow: the 'overflow-x', 'overflow-y', and 'overflow'
 	the [=used values=]
 	of all computed values other than ''visible''
 	is ''overflow/clip''.
+	Host languages should define [=UA style sheet=] rules
+	that apply a default value of ''overflow/clip'' to such elements
+	and set their 'overflow-clip-margin' to ''overflow-clip-margin/content-box''.
+
+	Note: Application of 'overflow' to [=replaced elements=] was added
+	to allow images to paint effects outside their effective layout box;
+	the recommended UA style sheet rules are to effect their original behavior as a default.
+	See discussion in <a href="https://github.com/w3c/csswg-drafts/issues/7058">Issue 7059</a>
+	and <a href="https://github.com/w3c/csswg-drafts/issues/7144">Issue 7144</a>.
+	This is a change from <a href="https://www.w3.org/TR/CSS2/">CSS2.1</a>.
 
 <h3 id="static-media">
 Overflow in Print and Other Static Media</h3>