关于background属性理解
/*background : background-color || background-image || background-repeat || background-attachment || background-position */
background: #000 url(../img/bg.png) no-repeat fixed 0 0 ;
/*background-origin : border | padding | content 用来决定 background-position 计算的参考位置*/
/*background-clip : border-box | padding-box | content-box | no-clip 用来确定背景的裁剪区域。*/
/*background-size :[ <length> | <percentage> | auto ]{1,2} | cover | contain 设置背景图片的大小*/
background-size: 100% 100%;
position: absolute;
width: 100%;
height: 100%;