CSS三角形提示框

/*CSS三角形提示框-向上*/
.tag_a{ width:406px; height:325px; border:1px solid #cccccc; position:absolute;z-index:3;background-color:#FFF;top:30px;left:25px;}
.tag_a em{border:0px;display:block; position:absolute;top:-18px;left:34px;width:0;font-size:0;height:0;line-height:0;border-left: 18px solid transparent;border-bottom: 18px solid #cccccc;border-right: 18px solid transparent;}
.tag_a b{border:0px;display:block; position:absolute;top:-17px;left:34px;width:0;font-size:0;height:0;line-height:0;border-left: 18px solid transparent;border-bottom: 18px solid #FFF;border-right: 18px solid transparent;
}
/*CSS三角形提示框-向左*/
.tag_b{ width:406px; height:325px; border:1px solid #cccccc; position:absolute;z-index:3;background-color:#FFF;top:30px;left:500px;}
.tag_b em{display:block; position:absolute;left:-18px;top:25px;width:0;font-size:0;height:0;line-height:0;border-top: 18px solid transparent;border-right: 18px solid #cccccc;border-bottom: 18px solid transparent;}
.tag_b b{display:block; position:absolute;left:-17px;top:25px;width:0;font-size:0;height:0;line-height:0;border-top: 18px solid transparent;border-right: 18px solid #FFF;border-bottom: 18px solid transparent;
}

<div class="tag_a">
    <em></em>
    <b></b>
</div>
<div class="tag_b">
    <em></em>
    <b></b>
</div>