一般来说，position：absolute之后跟left,top，这里为啥是width，height？

.nav-wrap nav a{                position: absolute;                width: 30px;                height: 30px;                background: #f44283;                text-align: center;                line-height: 30px;                text-decoration: none;                color: #fff;                border-radius: 3px;                text-shadow: 1px 1px 0px #000;                transform: translate(-50%,-50%);            }