[css-shapes] clarify behavior of empty polygon

Author: astearns

css-shapes/Overview.html

@@ -519,8 +519,13 @@ <h4 id=supported-basic-shapes><span class=secno>3.2.1. </span>Supported
     </ul>
 
     <p>The UA must close a polygon by connecting the last vertex with the
-     first vertex of the list.</p>
-    <br>
+     first vertex of the list.
+
+    <p>At least three vertices are required to define a polygon with an area
+     to contribute to a float area. Polygons with less than three vertices
+     (or with three or more vertices arranged to enclose no area) result in
+     an empty float area, which allows inline content to flow through all of
+     the float's box.
   </dl>
 
   <p>The values for corner radii on rectangle or inset-rectangle are
@@ -550,7 +555,7 @@ <h4 id=basic-shape-syntax><span class=secno>3.2.2. </span>Syntax of Basic
 
 ellipse() = ellipse( &lt;shape-arg&gt;, &lt;shape-arg&gt;, &lt;shape-arg&gt;, &lt;shape-arg&gt; )
 
-polygon() = polygon( [&lt;fill-rule&gt;,]? [&lt;shape-arg&gt;]{2}# )
+polygon() = polygon( [&lt;fill-rule&gt;,]? [&lt;shape-arg&gt;]{2} [&lt;shape-arg&gt;]{2}# )
 
 &lt;shape-arg&gt; = &lt;length&gt; | &lt;percentage&gt;
 

css-shapes/Overview.src.html

@@ -444,7 +444,16 @@ <h4 id="supported-basic-shapes">Supported Shapes</h4>
               <p>The UA must close a polygon 
                 by connecting the last vertex 
                 with the first vertex of the list.</p>
-              <br/>
+              <p>At least three vertices are required 
+                to define a polygon with an area 
+                to contribute to a float area. 
+                Polygons with less than three vertices 
+                (or with three or more vertices 
+                arranged to enclose no area) 
+                result in an empty float area, 
+                which allows inline content 
+                to flow through all 
+                of the float's box.</p>
           </dd>
       </dl>
 
@@ -479,7 +488,7 @@ <h4 id='basic-shape-syntax'>Syntax of Basic Shapes</h4>
 
 ellipse() = ellipse( &lt;shape-arg&gt;, &lt;shape-arg&gt;, &lt;shape-arg&gt;, &lt;shape-arg&gt; )
 
-polygon() = polygon( [&lt;fill-rule&gt;,]? [&lt;shape-arg&gt;]{2}# )
+polygon() = polygon( [&lt;fill-rule&gt;,]? [&lt;shape-arg&gt;]{2} [&lt;shape-arg&gt;]{2}# )
 
 &lt;shape-arg&gt; = &lt;length&gt; | &lt;percentage&gt;