[css-sizing-4] Add warning about possible errors in the aspect-ratio spec, than CSS2+flex+grid takes precedence.

Author: fantasai

css-sizing-4/Overview.bs

@@ -150,6 +150,17 @@ Aspect Ratios</h2>
 	for non-replaced elements,
 	and for altering the effective aspect ratio of replaced elements.
 
+	ISSUE: We are still working through the details of this section.
+	If there is any behavior specified here
+	that would cause [=replaced elements=] with a [=preferred aspect ratio=]
+	to behave differently than they would
+	under the requirements of the
+	<a href="https://www.w3.org/TR/CSS2/visudet.html">CSS2</a>,
+	<a href="https://www.w3.org/TR/css-flexbox-1/">Flex Layout</a>,
+	and <a href="https://www.w3.org/TR/css-grid-1/">Grid Layout</a>
+	specs combined (without this specification in effect),
+	<strong>this is an error and should be <a href="https://github.com/w3c/csswg-drafts/issues">reported</a> to the CSSWG</strong>.
+
 <h3 id="aspect-ratio">
 Preferred Aspect Ratios: the 'aspect-ratio' property</h3>