案例三角：
<!DOCTYPE html>
<html lang="en"><head><meta charset="UTF-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title><style>.box {margin: 100px auto;position: relative;height: 100px;width: 50px;background-color: pink;}.box span {position: absolute;right: 15px;top: -10px;height: 0;width: 0;/* 为了照顾兼容性 */line-height: 0;font-size: 0;border: 5px solid transparent;border-bottom-color: pink;}</style>
</head><body><div class="box"><span></span></div>
</body></html>
