[css-shapes-2] Make shape-padding consistent with shape-margin

- also accept percentage
- enforce non-negative values

css-shapes-2/Overview.bs

@@ -637,14 +637,21 @@ The 'shape-padding' Property</h4>
 
 	<pre class='propdef'>
 		Name: shape-padding
-		Value: <<length>>
-		Initial: none
+		Value: <<length-percentage [0,∞]>>
+		Initial: 0
 		Applies To: block-level elements
 		Inherited: no
-		Computed Value: the absolute length
+		Percentages: refer to the <a>inline size</a> of the containing block
+		Computed Value: computed <<length-percentage>> value
 		Animation type: by computed value
 	</pre>
 
+	<dl dfn-type="value" dfn-for="shape-padding">
+		<dt><dfn><<length-percentage [0,∞]>></dfn></dt>
+		<dd>
+			Sets the padding of the shape to the specified value.
+	</dl>
+
 	<div class="example">
 		<figure>
 			<img src="images/shape-padding.png" alt="Example of a shape-padding offset"/>