如何在css中设计tringle边框？

Question

使用Html5,css3,Bootstrap我想像这样设计这个div

我对css很新.所以请帮帮我这样做？

Answer 1

检查此代码段

.box {
    width:300px;
    position: relative;
    background: #fff;
    height:100px;
    top:50px;
    border: 4px solid #000;
}
.box:after, .box:before {
    bottom: 100%;
    left: 50%;
    border: solid transparent;
    content: " ";
    height: 0;
    width: 0;
    position: absolute;
    pointer-events: none;
}

.box:after {
    border-color: rgba(136, 183, 213, 0);
    border-bottom-color: #fff;
    border-width: 20px;
    margin-left: -20px;
}
.box:before {
    border-color: rgba(194, 225, 245, 0);
    border-bottom-color: #000;
    border-width: 26px;
    margin-left: -26px;
}

<div class="box"></div>