.box1{width:300px; height:300px; background:#CCC; position:relative; top:100px;left:100px }
.to-top{display:block; overflow:hidden; width:0px; height:0px; border:6px solid #ccc; border-color:#ccc #fff; border-width:0 6px 6px 6px; position:absolute; left:30px; top:0;margin-top:-6px;}
.to-right{display:block; overflow:hidden; width:0px; height:0px; border:6px solid #ccc; border-color:#fff #ccc; border-width:6px 0 6px 6px; position:absolute; right:0px; top:30px;margin-right:-6px;}
.to-bottom{display:block; overflow:hidden; width:0px; height:0px; border:6px solid #ccc; border-color:#ccc #fff; border-width:6px 6px 0 6px; position:absolute; right:30px; bottom:0;margin-bottom:-6px;}
.to-left{display:block; overflow:hidden; width:0px; height:0px; border:6px solid #ccc; border-color:#fff #ccc; border-width:6px 6px 6px 0 ; position:absolute; left:0; bottom:30px;margin-left:-6px;}
H5
<div class="box1"><em class="to-top"></em><em class="to-right"></em><em class="to-bottom"></em><em class="to-left">
效果：
效果图.png
