只有一张背景图怎么适配各尺寸的屏幕

背景图居中铺满，保持宽高比，不用显示全部时：

1 background-image: url("background.jpg");
2 background-repeat: no-repeat;
3 background-size: cover;
4 background-position: center;

背景图铺满且不允许裁剪时:

background-image: url("background.jpg");
background-repeat: no-repeat;
background-size: 100% 100%;
background-position: center;

保持宽高比，且不允许裁剪时（不能铺满容器）:

background-image: url("background.jpg");
background-repeat: no-repeat;
background-size: contain;
background-position: center;