<style>
#triangle-down { width: 0;
height: 0;
border-left: 50px solid transparent;
border-right: 50px solid transparent;
border-top: 100px solid red;
}</style>
<div id="triangle-down"></div>
或
.triangle {
display: inline-block;
width: 0;
height: 0;
line-height: 0;
border: 20px solid transparent;
border-top-color: #000;
border-bottom-width: 0;
}