SOURCE

 <div class="wb_arrow"></div>
 <div class="arrow"></div>

  /*简单*/
            .wb_arrow{
                border-right: 2px solid #343c99;
                border-top: 2px solid #343c99;
                height: 10px;
                width: 10px;
                margin:50px auto 0;
                transform: rotate(0deg);
                -webkit-transform: rotate(0deg);
                /*不加这两个属性三角会比上一个略丑, 大家可以试一下*/
                border-left: 2px solid transparent;
                border-bottom: 2px solid transparent;
            }

.arrow {
                width: 20px;
                height: 4px;
                margin: 0 auto 7px;
                border-left: 4px solid transparent;
                border-right: 4px solid transparent;
                border-bottom: 4px solid #343c99;
                transform: rotate(45deg);
                transform-origin: left;
            }
            
            .arrow:after {
                content: '';
                display: block;
                width: 100%;
                height: 100%;
                border-left: 4px solid transparent;
                border-right: 4px solid transparent;
                border-top: 4px solid #343c99;
                position: absolute;
                right: -10px;
                top: -14px;
                transform: rotate(90deg);
                transform-origin: bottom;
            }