实现过程：

正常的border

<div class="box"></div>
.box {
     background: #ddd;
     width: 100px;
     height: 100px;
     border-top: 20px solid yellow;
     border-right: 20px solid red;
     border-bottom: 20px solid black;
     border-left: 20px solid blue;
}

高度和宽度减小后的border

正常border表现为梯形，当box的高度和宽度减小至0时，border则为三角形

.box {
        background: #ddd;
        width: 0;
        border-top: 20px solid yellow;
        border-right: 20px solid red;
        border-bottom: 20px solid black;
        border-left: 20px solid blue;
}

接下通过设置其他三个border的背景为透明则可以实现三角形

.box {
        width: 0;
        border-top: 20px solid yellow;
        border-right: 20px solid transparent;
        border-bottom: 20px solid transparent;
        border-left: 20px solid transparent;
}

.box {
        width: 0;
        border-top: 20px solid transparent;
        border-right: 20px solid red;
        border-bottom: 20px solid transparent;
        border-left: 20px solid transparent;
}

.box {
        width: 0;
        border-top: 20px solid transparent;
        border-right: 20px solid transparent;
        border-bottom: 20px solid black;
        border-left: 20px solid transparent;
}

.box {
        width: 0;
        border-top: 20px solid transparent;
        border-right: 20px solid transparent;
        border-bottom: 20px solid transparent;
        border-left: 20px solid blue;
}

 

参考：https://www.cnblogs.com/youhong/p/6530575.html