[css-shapes] clarify empty circle/ellipse for w3c#850

Author: astearns

css-shapes/Overview.bs

@@ -16,6 +16,12 @@ Abstract: CSS Shapes describe geometric shapes for use in CSS. For Level 1, CSS
 Link Defaults: css21 (property) margin
 </pre>
 
+<pre class='link-defaults'>
+spec:css-masking-1; type: value
+	text: nonzero
+	text: evenodd
+</pre>
+
 <style type="text/css">
 	.singleImgExample {
 		display: block;
@@ -350,6 +356,9 @@ Supported Shapes</h3>
 					<strong>r</strong>, the radius
 					of the circle.
 					Negative values are invalid.
+					A radius of zero defines a shape
+					enclosing no area, 
+					which results in an <a>empty float area</a>.
 					A percentage value here
 					is resolved from the used width and height
 					of the <a>reference box</a> as <br>
@@ -375,6 +384,9 @@ Supported Shapes</h3>
 					of the ellipse,
 					in that order.
 					Negative values for either radius are invalid.
+					If either radius is zero this defines a shape
+					enclosing no area, 
+					which results in an <a>empty float area</a>.
 					Percentage values here are resolved
 					against the used width (for the rx value)
 					and the used height (for the ry value)
@@ -1054,6 +1066,7 @@ Change Log</h2>
 Since <a href="https://www.w3.org/TR/2014/CR-css-shapes-1-20140320/">March 20th 2014</a></h3>
 	<ul>
 		<li>Clarified shape-margin computed value</li>
+		<li>Clarified empty circles and ellipses for <a href="https://github.com/w3c/csswg-drafts/issues/850">issue #850</a></li>
 	</ul>
 
 <h3 class="no-num" id="20140211">