背景图片高度是1400像素，网页高度是1900像素，到浏览到第三屏时背景图片固定。这个怎么写，这问题不太好描述，不知道能不能看明白。
用jquery scroll()来侦测是否浏览到第三屏，然后添加一个class，使得background-position, fixed.
jQuery(window).scroll(function(){ if(jQuery(window).scrollTop() > 1399){ jQuery('html').addClass('scrolled'); }else{ jQuery('html').removeClass('scrolled'); }});html { background:url(image.jpg); background-repeat: no-repeat; background-position: top left;}html.scrolled { background-attachment:fixed; background-position: bottom left;} 算是笨方法 你参考
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>无标题文档</title><style type="text/css">*{padding:0; margin:0;}body{background:url(http://b.hiphotos.baidu.com/album/w%3D2048/sign=19d4e347c2fdfc03e578e4b8e0078694/83025aafa40f4bfb2ac58bed024f78f0f7361875.jpg) 50% 0 no-repeat;}body.over{background:url(http://b.hiphotos.baidu.com/album/w%3D2048/sign=19d4e347c2fdfc03e578e4b8e0078694/83025aafa40f4bfb2ac58bed024f78f0f7361875.jpg) 50% bottom no-repeat fixed;}.box{height:5000px;}</style></head><body><div class="box"></div><script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script><script type="text/javascript">$(function(){ var winHeight=$(window).height(); var maxHeight=3355-winHeight; //3355为背景图片高度 $(window).scroll(function(){ if($(this).scrollTop()>maxHeight){ $("body").addClass("over"); }else{ $("body").removeClass("over"); } });});</script></body></html>
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