title: Add Placeholder text to a text field in a form
body: <p>Hi all i am trying to add placeholder to a skin i am building on roundcube.
  i want it to say username for username and password for password.
  i know its possible using html but the issue is i dont have acess to the form so i can add placeholder text to the html.
  I think the only way will be with js or css but as of yet not found a working solution and hoping someone here can help.
  Here is the form html it has:</p>
  
  <pre><code>    &lt;html&gt;
  &lt;input type="hidden" name="_task" value="login"&gt;&lt;input type="hidden" name="_action" value="login"&gt;&lt;input type="hidden" name="_timezone" id="rcmlogintz" value="_default_"&gt;&lt;input type="hidden" name="_url" id="rcmloginurl" value=""&gt;&lt;table&gt;&lt;tbody&gt;&lt;tr&gt;&lt;td class="title"&gt;&lt;label for="rcmloginuser"&gt;Username&lt;/label&gt;
  &lt;/td&gt;
      &lt;td class="input"&gt;&lt;input name="_user" id="rcmloginuser" required="required" size="40" autocapitalize="off" autocomplete="off" type="text"&gt;&lt;/td&gt;
      &lt;/tr&gt;
      &lt;tr&gt;&lt;td class="title"&gt;&lt;label for="rcmloginpwd"&gt;Password&lt;/label&gt;
      &lt;/td&gt;
      &lt;td class="input"&gt;&lt;input name="_pass" id="rcmloginpwd" required="required" size="40" autocapitalize="off" autocomplete="off" type="password"&gt;&lt;/td&gt;
      &lt;/tr&gt;
      &lt;/tbody&gt;
      &lt;/html&gt;
  </code></pre>
  
  <p>Many thanks all.</p>