下面的CSS是什么意思? 如何在下面的CSS中加鼠标经过变背景图片的JS代码?
.top_menu{ position:absolute; bottom:0; _bottom:-1px; right:10px; height:28px;}
.top_menu ul li{ float:left; display:inline-block; margin:0 2px;}
.top_menu ul li a{ color:#FFF;background:url(../images/top_menu_bg.jpg) -97px top no-repeat; width:97px; line-height:28px; text-align:center; display:inline-block; }
.top_menu ul li.curr a{ background:url(../images/top_menu_bg.jpg) left top no-repeat; font-weight:bold}