="element" ng-class="{'dn-fade-up':dn_fade_up}">
        ...
      </div>
    </section>

    <section>
      <h2>.dn-fade-down</h2>
      <button ng-click="dn_fade_down=!dn_fade_down">Toggle Class</button>
      <div class="element" ng-class="{'dn-fade-down':dn_fade_down}">
        ...
      </div>
    </section>

    <section>
      <h2>.dn-fade-left</h2>
      <button ng-click="dn_fade_left=!dn_fade_left">Toggle Class</button>
      <div class="element" ng-class="{'dn-fade-left':dn_fade_left}">
        .