Add a test for the [hidden] attribute

Author: necolas

demo.html

@@ -38,6 +38,8 @@ <h3>Heading 3</h3>
         <h4>Heading 4</h4>
         <h5>Heading 5</h5>
         <h6>Heading 6</h6>
+
+        <p hidden>This should be hidden in all browsers, apart from IE6</p>
 
         <section>
             <h1>Section Heading 1</h1>