Background背景的属性如下：
●background-color:#f00;
●background-image:url(background.gif);
●background-repeat:no-repeat;
●background-attachment:fixed;
●background-position:0 0;
●background-image:url(background.gif);
●background-repeat:no-repeat;
●background-attachment:fixed;
●background-position:0 0;
可以缩写为一句：background:#f00 url(background.gif) no-repeat fixed 0 0;
语法是background:color image repeat attachment position;
你可以省略其中一个或多个属性值，如果省略，该属性值将用浏览器默认值，默认值为：
●color: transparent
●image: none
●repeat: repeat
●attachment: scroll
●position: 0% 0%
●image: none
●repeat: repeat
●attachment: scroll
●position: 0% 0%
