一、用伪元素做平行四边形
.button { position: relative; width: 100px; height: 100px; background-color: rgba(0,0,0,.0); margin: 100px; }
.button::before {content: '';position: absolute;top: 0;right: 0;bottom: 0;left: 0;z-index: -1;background: #58a;transform: skew(45deg);}