background-attachment

background-attachment CSS 属性决定背景图像的位置是在视口内固定，或者随着包含它的区块滚动。

示例

/* 关键 属性值 */
background-attachment: scroll;
background-attachment: fixed;
background-attachment: local;

/* 全局 属性值 */
background-attachment: inherit;
background-attachment: initial;
background-attachment: unset;

浏览器支持

语法：

取值

• fixed此关键字表示背景相对于视口固定。即使一个元素拥有滚动机制，背景也不会随着元素的内容滚动。
• local此关键字表示背景相对于元素的内容固定。如果一个元素拥有滚动机制，背景将会随着元素的内容滚动，并且背景的绘制区域和定位区域是相对于可滚动的区域而不是包含他们的边框。
• scroll此关键字表示背景相对于元素本身固定，而不是随着它的内容滚动（对元素边框是有效的）。

例子

//CSS

p {
  background-image: url("https://mdn.mozillademos.org/files/12057/starsolid.gif");
  background-attachment: fixed;
}

//html
 there were doors all round the hall, but they were all locked; and when
  alice had been all the way down one side and up the other, trying every
  door, she walked sadly down the middle, wondering how she was ever to
  get out again.

多背景图支持

此属性支持多张背景图片。你可以用逗号分隔来为每一张背景图片指定不同的。每一张背景图片顺序对应相应的attachment 类型。

//CSS
p {
  background-image: url("https://mdn.mozillademos.org/files/12057/starsolid.gif"), url("https://mdn.mozillademos.org/files/12059/startransparent.gif");
  background-attachment: fixed, scroll;
  background-repeat: no-repeat, repeat-y;
}

//html
 there were doors all round the hall, but they were all locked; and when
  alice had been all the way down one side and up the other, trying every
  door, she walked sadly down the middle, wondering how she was ever to
  get out again.

  suddenly she came upon a little three-legged table, all made of solid
  glass; there was nothing on it except a tiny golden key, and alice's
  first thought was that it might belong to one of the doors of the hall;
  but, alas! either the locks were too large, or the key was too small,
  but at any rate it would not open any of them. however, on the second
  time round, she came upon a low curtain she had not noticed before, and
  behind it was a little door about fifteen inches high: she tried the
  little golden key in the lock, and to her great delight it fitted!

body{
	width:150px;
	height:80px;
	background-image:url("images/logo.png");
	background-attachment:fixed;
	background-repeat:no-repeat;
}
div{
	height:2000px;
} 

效果图：

不管页面怎么向下滚动，图片的位置固定那不动

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