**完整简写：**
background:
[image]
[position] / [size]
[repeat]
[attachment]
[origin]
[clip]
[color];
**示例：**
background:
url('img.jpg')
center / cover
no-repeat
fixed;
**多层简写：**
background:
linear-gradient(...) center/cover,
url('img.jpg') center/cover no-repeat;
