css实现实心三角形

1、html

<body style="flex-direction: column; height: 600px;display: flex; justify-content: center; align-items: center;">
    <!--向上的三角形-->
    <div class="up"></div></br>
    <!--向下的三角形-->
    <div class="down"></div></br>
    <!--向左的三角形-->
    <div class="left"></div></br>
    <!--向右的三角形-->
    <div class="right"></div>
</body>

2、css

/*想获取朝那边三角形,给相反的一边设置颜色,font-size: 0;line-height: 0;可以兼容ie6*/
.up {
  width: 0;
  height: 0;
  border-left: 20px solid transparent;
  border-right: 20px solid transparent;
  border-bottom: 20px solid #f00;
  font-size: 0;
  line-height: 0;
}
.down {
  width: 0;
  height: 0;
  border-left: 20px solid transparent;
  border-right: 20px solid transparent;
  border-top: 20px solid #f00;
  font-size: 0;
  line-height: 0;
}
.left {
  width: 0;
  height: 0;
  border-top: 20px solid transparent;
  border-right: 20px solid #f00;
  border-bottom: 20px solid transparent;
  font-size: 0;
  line-height: 0;
}
.right {
  width: 0;
  height: 0;
  border-top: 20px solid transparent;
  border-left: 20px solid #f00;
  border-bottom: 20px solid transparent;
  font-size: 0;
  line-height: 0;
}

3、效果图