Nonpositive lengths for 'perspective' should be invalid

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=16183.

Author: ayg

css3-transforms/ChangeLog

@@ -1,3 +1,7 @@
+2012-03-12 ayg@aryeh.name
+    Nonpositive lengths for 'perspective' should be invalid
+    Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=16183.
+
 2012-03-12 ayg@aryeh.name
     Make perspective-origin syntax match transform-origin
     Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=16064.

css3-transforms/Overview.html

@@ -1723,8 +1723,8 @@ <h2 id=perspective-property><span class=secno>10. </span> The &lsquo;<a
   </table>
 
   <p> If the value is &lsquo;<code class=css><code
-   class=css>none</code></code>&rsquo;, less than or equal to 0 no
-   perspective transform is applied.
+   class=css>none</code></code>&rsquo;, no perspective transform is applied.
+   Lengths must be positive.
 
   <p> The use of this property with any value other than &lsquo;<code
    class=css><code class=css>none</code></code>&rsquo; establishes a stacking

css3-transforms/Transforms.src.html

@@ -1320,8 +1320,8 @@ <h2 id="perspective-property">
               </table>
 
               <p>
-                If the value is '<code class="css">none</code>', less than or equal to 0 no
-                perspective transform is applied.
+                If the value is '<code class="css">none</code>', no perspective
+                transform is applied.  Lengths must be positive.
               </p>
               <p>
                 The use of this property with any value other than '<code class="css">none</code>' establishes a