rm :invalid styling from docs

It was removed from the code in 90d4343  :-/

Author: cvrebert

docs/css.html

@@ -1380,21 +1380,6 @@ <h3 id="forms-input-focus">Input focus</h3>
     </form>
 {% highlight html %}
 <input id="focusedInput" type="text" value="This is focused...">
-{% endhighlight %}
-
-    <h3 id="forms-invalid-inputs">Invalid inputs</h3>
-    <p>Style inputs via default browser functionality. Specify a <code>type</code>, add the <code>required</code> attribute if the field is not optional, and (if applicable) specify a <code>pattern</code>.</p>
-
-    <div class="bs-callout">
-      <h4>Cross-browser compatibility</h4>
-      <p>Invalid inputs are styled via the <code>:invalid</code> CSS selector, which is not supported by Internet Explorer 9 and below.</p>
-    </div>
-
-    <form class="bs-example">
-      <input type="email" placeholder="[email protected]" required>
-    </form>
-{% highlight html %}
-<input type="email" placeholder="[email protected]" required>
 {% endhighlight %}
 
     <h3 id="forms-disabled-inputs">Disabled inputs</h3>