[css-break-3] Require zero-sized fragments that fit to stay on previo…

…us fragmentainer. #1529

css-break-3/Overview.bs

@@ -793,6 +793,17 @@ Optimizing Unforced Breaks</h3>
     </ul>
   </div>
 
+  Additionally, CSS imposes one requirement:
+  a zero-sized <a>box fragment</a>,
+  since it does not take up space,
+  must appear on the earlier side of a <a>fragmentation break</a>
+  if it is able to fit within the <a>fragmentainer</a>.
+
+  Note: A zero-sized <a>box fragment</a> can be pushed
+  to the next <a>fragmentainer</a>
+  if it is placed immediately after content
+  that itself overflows the <a>fragmentainer</a>.
+
 <h2 id="breaking-boxes">
 Box Model for Breaking</h2>