HTMLPurifier allow class attribute

How can i allow "class" in HTMLPurifier? I am trying to purify this:

 <div class="txt_r" id="test">Blah</div>

And i get:

 <div id="test">Blah</div>

Why class is dissapeared? I am using next config:

 $config->set('Attr.EnableID', true);
 $config->set('CSS.Trusted', true);
 $config->set('HTML.AllowedAttributes', 'style, src, class');