[css-sizing] Address Amelia's concerns in w3c#1609.

Author: tabatkins

css-sizing-3/Overview.bs

@@ -393,8 +393,10 @@ Intrinsic Sizes</h3>
 	is the size it would have as an
 	''width/auto''-sized (in that axis) float in an infinitely-sized containing block.
 	(In other words, the maximum size it has when sized as “shrink-to-fit”.)
-	If this size would depend on the size of the containing block
-	(as it does for images with an intrinsic aspect ratio but no intrinsic size)
+	For replaced elements,
+	if this size would depend on the size of the containing block
+	(as it does for images with an intrinsic aspect ratio but no intrinsic size,
+	see <a href="https://www.w3.org/TR/CSS2/visudet.html#inline-replaced-width">CSS2&sect;10.3.2</a>)
 	and therefore can't be calculated against an infinitely-sized containing block
 	then:
 
@@ -420,12 +422,23 @@ Intrinsic Sizes</h3>
 				as needed.
 	</dl>
 
+	Since a block-level or inline-level replaced element
+	whose 'height' or 'width' <a>behaves as auto</a>
+	is effectively defined to use its <a>max-content size</a>
+	(<a href="https://www.w3.org/TR/CSS2/visudet.html#inline-replaced-width">CSS2&sect;10.3.2</a>),
+	this specification applies the rules above
+	to the undefined case of a replaced element
+	whose 'height' and 'width' both <a>behave as auto</a>.
+
 	Note: This specification does not define how to determine
 	the size of a float.
 	Please refer to [[CSS2]],
 	the relevant CSS specification for that display type,
 	and/or existing implementations
 	for further details.
+	A future specification will define this in detail,
+	replacing the CSS2 “definition”,
+	such as it is.
 
 <h3 id="intrinsic-contribution">
 Intrinsic Contributions</h3>