[css-display] Add issue

Author: fantasai

css-display/Overview.bs

@@ -799,6 +799,11 @@ Appendix A: Glossary</h2>
 
 			Note: some formatting contexts inhibit floating,
 			so that an element with ''float: left'' is not necessarily <a>out-of-flow</a>.
+
+			Issue: CSS2.1 defines the root element as out-of-flow.
+			This <a href="https://lists.w3.org/Archives/Public/www-style/2015Aug/0332.html">seems to me</a>
+			to be more likely to cause problems in our specs than not.
+			Should we change that definition or copy it over?
 	</dl>
 
 	See [[!CSS2]] <a href="http://www.w3.org/TR/CSS2/visuren.html">Chapter 9</a> for a fuller definition of these terms.