背景大小 | background-size (Backgrounds & Borders) - CSS 中文开发手册

background-size CSS属性指定一个元素的背景图像的大小。图像可以保留其自然大小，拉伸到一个新的大小，或约束，以适应可用空间，同时保持其固有的比例。

/* Keyword values */
background-size: cover;
background-size: contain;

/* One-value syntax */
/* the width of the image (height becomes ‘auto‘) */
background-size: 50%;
background-size: 3.2em;
background-size: 12px;
background-size: auto;

/* Two-value syntax */
/* first value: width of the image, second value: height */
background-size: 50% auto;
background-size: 3em 25%;
background-size: auto 6px;
background-size: auto auto;

/* Multiple backgrounds */
background-size: auto, auto; /* Not to be confused with `auto auto` */
background-size: 50%, 25%, 25%;
background-size: 6px, auto, contain;

/* Global values */
background-size: inherit;
background-size: initial;
background-size: unset;