CSS：展开折叠icon效果

分析：CSS 展开和折叠使用不同的icon类，这里用到属性content

展开前

<i class="glyphicon glyphicon-chevron-up" aria-hidden="true"></i>

.glyphicon {
    position: relative;
    top: 1px;
    display: inline-block;
    font-family: 'Glyphicons Halflings';
    font-style: normal;
    font-weight: normal;
    line-height: 1;
    -webkit-font-smoothing: antialiased;}

展开后

.glyphicon-chevron-down:before {content: "\e113";}