css3 画三角形

/*箭头向上*/

.arrow-up {
width:0;
height:0;
border-left:20px solid transparent;
border-right:20px solid transparent;
border-bottom:20px solid #000;
}

/*箭头向下*/
.arrow-down {
width:0;
height:0;
border-left:20px solid transparent;
border-right:20px solid transparent;
border-top:20px solid #0066cc;
}

/*箭头向左*/
.arrow-left {
width:0;
height:0;
border-top:20px solid transparent;
border-bottom:20px solid transparent;
border-right:20px solid yellow;
}

/*箭头向右*/
.arrow-right {
width:0;
height:0;
border-top:20px solid transparent;
border-bottom: 20px solid transparent;
border-left: 20px solid green;
}

<div class="arrow-up">
<!--向上的三角-->
</div>
<div class="arrow-down">
<!--向下的三角-->
</div>
<div class="arrow-left">
<!--向左的三角-->
</div>
<div class="arrow-right">
<!--向右的三角-->
</div>