当鼠标移动到链接时链接改变背景色，点击时链接颜色有变化，当点击其他链接时上次点击的链接又变为原来的颜色，以此区分用户正在访问的是哪一个链接。-Link when the mouse moves to change the background color of the link, clicked the link color change, when you click another link, click on the link and become the last original color, this distinction is what the user is accessing a link.