CSS: dropdown menu css

gistfile1

/***********************************************
NAV CSS
***********************************************/
#nav_holder { background-color: #000; width: 100%; height: 35px; background-image: url(images/stripe_bg.jpg); background-repeat: repeat-x;  margin: 0 0 10px 0;}

#main_nav{ float: left; margin: 0px 0px 10px 0px; height: 35px; color: #fff; width: 650px; /*background-color: #000;*/ }
#main_nav ul { font-size: 14px; list-style: none; }
#main_nav ul li { display: block; position: relative; float: left; z-index: 99999; }
#main_nav li ul { display: none; }
#main_nav ul li a {  display: block; text-decoration: none; color: #ffffff; padding: 10px 15px; margin-left: 1px; white-space: nowrap; border-right: 1px solid #444; }
#main_nav ul li a:hover { background-color: #444; }
#main_nav li:hover ul { display: block; position: absolute; background-color: #444;}
#main_nav li:hover li {  float: none; font-size: 14px; }
#main_nav li:hover a { }
#main_nav li:hover li a:hover { background-color: #000; }
/***********************************************
/NAV CSS
***********************************************/