这里ul的class定义了吗?
<ul class="clearfix unstyled drop-menu">
<li><a href="#" class="three-d">
Html5
<span class="three-d-box"><span class="front">Html5</span><span class="back">Html5</span></span>
</a></li>
<li><a href="#" class="three-d">
Css3
<span class="three-d-box"><span class="front">Css3</span><span class="back">Css3</span></span>
</a></li>
<li><a href="#" class="three-d">
JavaScript
<span class="three-d-box"><span class="front">JavaScript</span><span class="back">JavaScript</span></span>
</a></li>
<li><a href="#" class="three-d">
Videogames
<span class="three-d-box"><span class="front">Videogames</span><span class="back">Videogames</span></span>
</a></li>
</ul>