背景：圣诞节又到了，拿什么来哄女朋友开心？
项目结构：
html:
<!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>CSS3圣诞树雪人下雪动画效果</title><style type="text/css">
body{margin:0;padding:0;list-style-type:none;}
.header{margin: 0 0 30px;background: url(css/img/header-bg.png);background: url(css/img/snow-bg.png) repeat-y center, url(
