background样式
background-image: url("a.jpg");
/默认状态下是平铺的/
background-repeat: no-repeat;
/背景不平铺/
background-color: red;
/背景图片比背景颜色层级高/
background-repeat: repeat-x;
/背景水平平铺/
background-repeat: repeat-y;
/背景垂直平铺/
background-position: 10px 10px;
/背景定位/
第一个参数水平，第二个参数垂直
left、center、right top、center、bottom

background-attachment: fixed;
/背景固定/
background-attachment: scroll;
/默认样式滚动/
合写
background: red url("a.jpg") no-repeat center center;