[css-sizing] Specify how to handle %s in width/max-width/etc when finding min-content contribution on replaced elements. Addresses #765.

Author: tabatkins

css-sizing-3/Overview.bs

@@ -424,6 +424,20 @@ Intrinsic Contributions</h3>
 	that contains only that box,
 	if that hypothetical float's containing block is zero-sized/infinitely-sized.
 
+	However, in the case of a [=replaced element|replaced=] box with a percentage-based 'width'/'max-width'/'height'/'max-height',
+	the percentage is resolved to zero
+	when calculating the <a>min-content contribution</a> in the corresponding axis.
+
+	<div class=example>
+		For example,
+		an <{input}> assigned ''width: calc(50% + 50px)''
+		has a <a>min-content contribution</a> of ''50px'',
+		plus any horizontal margin/border/padding.
+	</div>
+
+	Note: We are not 100% sure if zeroing out a percentage 'max-width' on form controls is web-compatible.
+	See <a href="https://github.com/w3c/csswg-drafts/issues/765">Issue 765</a>.
+
 	Note: This specification does not define how to determine these sizes.
 	Please refer to [[CSS2]],
 	the relevant CSS specification for that display type,