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background-size 设置背景图片大小。

/* 关键字 */
background-size: cover
background-size: contain

/* 一个值: 这个值指定图片的宽度,图片的高度隐式的为auto */
background-size: 50%
background-size: 3em
background-size: 12px
background-size: auto

/* 两个值 */
/* 第一个值指定图片的宽度,第二个值指定图片的高度 */
background-size: 50% auto
background-size: 3em 25%
background-size: auto 6px
background-size: auto auto

/* 逗号分隔的多个值:设置多重背景 */
background-size: auto, auto     /* 不同于background-size: auto auto */
background-size: 50%, 25%, 25%
background-size: 6px, auto, contain

/* 全局属性 */
background-size: inherit;
background-size: initial;
background-size: unset;


初始值auto auto
适用元素all elements. It also applies to ::first-letter and ::first-line.
Percentagesrelative to the background positioning area
计算值as specified, but with relative lengths converted into absolute lengths
Animation typerepeatable list of simple list of length, percentage, or calc
正规顺序the unique non-ambiguous order defined by the formal grammar


The background size for a single image can be specified in one of three ways:

  • using the keyword contain
  • using the keyword cover
  • using width and height values

To specify a size using width and height, you can supply one or two values:

  • If only one value is given it sets the width, with the height set to auto.
  • If two values are given, the first sets width and the second sets height.

Each value can be a <length>, a <percentage>, or auto.

For example:

background-size: contain;

background-size: 50%;
background-size: 3em;

background-size: auto 1em;
background-size: 50% 25%;

To specify background size for more than one image, supply multiple sizes, separated by commas:

background-size: 50% 25%, contain, 3em;


<length> 值,指定背景图片大小,不能为负值。
<percentage> 值,指定背景图片相对背景区(background positioning area)的百分比。背景区由background-origin设置,默认为盒模型的内容区与内边距,也可设置为只有内容区,或者还包括边框。如果attachmentfixed,背景区为浏览器可视区(即视口),不包括滚动条。不能为负值。
缩放背景图片以完全覆盖背景区,可能背景图片部分看不见。A keyword that is the inverse of contain. Scales the image as large as possible and maintains image aspect ratio (image doesn't get squished). The image "covers" the entire width or height of the container. When the image and container have different dimensions, the image is clipped either left/right or top/bottom.
缩放背景图片以完全装入背景区,可能背景区部分空白。A keyword that scales the image as large as possible and maintains image aspect ratio (image doesn't get squished). Image is letterboxed within the container. When the image and container have different dimensions, the empty areas (either top/bottom of left/right) are filled with the background-color.


Gecko 8.0 note
(Firefox 8.0 / Thunderbird 8.0 / SeaMonkey 2.5)

This behavior changed in Gecko 8.0 (Firefox 8.0 / Thunderbird 8.0 / SeaMonkey 2.5). Before this, gradients were treated as images with no intrinsic dimensions, with an intrinsic proportion identical to that of the background positioning area.

-moz-element 生成的背景图片,(which actually match an element) are currently treated as images with the dimensions of the element, or of the background positioning area if the element is SVG, with the corresponding intrinsic proportion.

Note: This is not the currently-specified behavior, which is that the intrinsic dimensions and proportion should be those of the element in all cases.


如果指定了 background-size 的两个值并且不是auto
autoauto auto:
图像如果有两个长度,则按这个尺寸。如果没有固有尺寸与固有比例,则按背景区的大小。如果没有固有尺寸但是有固有比例, 效果同 contain。如果有一个长度与比例,则由此长度与比例计算大小。如果有一个长度但是没有比例,则使用此长度与背景区相应的长度。
一个为 auto 另一个不是auto:

注意,对于没有固有尺寸或固有比例的矢量图不是所有的浏览器都支持。特别注意测试Firefox 7- 与Firefox 8+,以确定不同之处能否接受。

Formal syntax

如何阅读 CSS 语法。


<bg-size> = [ <length-percentage> | auto ]{1,2} | cover | contain

<length-percentage> = <length> | <percentage>


background-size: cover 演示 与  background-size: contain 演示 在新窗口打开,这样你可以看到当背景区大小变化时 containcover 是怎样的。 系列演示:background-size 及其与background-*属性的关联  很好的说明了单独使用 background-size 及与其它属性共同使用。


如果用渐变作为背景并且对它使用了background-size ,最好不要只用一个auto, 或者只指定一个宽度值 (例如 background-size: 50%)。对这两种情况 Firefox 8有所改变, 并且目前各浏览器表现不一致,不是全部浏览器都完全支持 CSS3 background-size 规范CSS3 Image Values gradient 规范

.bar {
       width: 50px; height: 100px;
       background-image: gradient(...);

       /* 不推荐 */
       background-size: 25px;
       background-size: 50%;
       background-size: auto 50px;
       background-size: auto 50%;

       /* 可行 */
       background-size: 25px 50px;
       background-size: 50% 50%;

特别不推荐对渐变px与auto一起用, 因为Firefox 8之前不能重复渲染,并且对于没有实现Firefox 8渲染特性的浏览器,不知道指定了背景的元素的确切大小。


规范 状态 说明
CSS Backgrounds and Borders Module Level 3
Candidate Recommendation Initial definition


FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support

1 -webkit- 1



3.6 -moz-

49 -webkit-



9.5 -o- 2


3 -webkit- 1


<contain> and <cover></cover></contain>3123.69104.1
SVG image as background44128931 ?
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support2.3 Yes Yes

4 -moz-

49 -webkit-


Yes Yes ?
<contain> and <cover></cover></contain> ? ? Yes Yes ? Yes ?
SVG image as background ? ? Yes8 Yes ? ?

1. WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords.

2. Opera 9.5's computation of the background positioning area is incorrect for fixed backgrounds. Opera 9.5 also interprets the two-value form as a horizontal scaling factor and, from appearances, a vertical clipping dimension. This has been fixed in Opera 10.

Polyfills and Workarounds

For Internet Explorer prior IE8

Though Internet Explorer 8 doesn't support the background-size property, it is possible to emulate some of its functionality using the non-standard -ms-filter property:

-ms-filter: "progid:DXImageTransform.Microsoft.AlphaImageLoader(src='path_relative_to_the_HTML_file', sizingMethod='scale')";

This simulates the value cover.

In Firefox 3.5

While this property was added in Firefox 3.6, it is possible to stretch a image fully over the background in Firefox 3.5 by using -moz-border-image.

.foo {
  background-image: url(bg-image.png);

  -webkit-background-size: 100% 100%;           /* Safari 3.0 */
     -moz-background-size: 100% 100%;           /* Gecko 1.9.2 (Firefox 3.6) */
       -o-background-size: 100% 100%;           /* Opera 9.5 */
          background-size: 100% 100%;           /* Gecko 2.0 (Firefox 4.0) and other CSS3-compliant browsers */
  -moz-border-image: url(bg-image.png) 0;    /* Gecko 1.9.1 (Firefox 3.5) */



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