[css-display] Be a little looser about what counts as abspos, because css-lists defines as out-of-flow. Fixes #1458

Author: fantasai

css-display/Overview.bs

@@ -996,7 +996,7 @@ Appendix A: Glossary</h2>
 		<dt><dfn>in-flow</dfn>
 		<dd>
 			A box is <a>out-of-flow</a> if it is floated (via 'float')
-			or absolutely-positioned (via ''absolute'' or ''position/fixed'' positioning).
+			or absolutely-positioned (via 'position').
 			A box is <a>in-flow</a> if it is not <a>out-of-flow</a>.
 
 			Note: some formatting contexts inhibit floating,