<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;}