start documenting html5 invalid form fields

docs/templates/pages/base-css.mustache

@@ -1506,6 +1506,15 @@
           </form>
 <pre class="prettyprint linenums">
 &lt;input class="input-xlarge" id="focusedInput" type="text" value="{{_i}}This is focused...{{/i}}"&gt;
+</pre>
+
+          <h3>{{_i}}Required inputs{{/i}}</h3>
+          <p>{{_i}}Style inputs via default browser functionality with <code>:valid</code>.{{/i}}</p>
+          <form class="bs-docs-example form-inline">
+            <input class="span4" type="text" value="{{_i}}Required input{{/i}}" required>
+          </form>
+<pre class="prettyprint linenums">
+&lt;input class="span4" type="text" placeholder="{{_i}}Required input{{/i}}" required&gt;
 </pre>
 
           <h3>{{_i}}Disabled inputs{{/i}}</h3>