很多主题都有这样的菜单。我通常使用的CSS（我忽略了颜色等视觉元素）：

（假设容器是div，其类为.nav)

/* 1st level items are layed out horizontally */
.nav li{
  position:relative;
  float:left;
}

/* level 2+ */
.nav ul ul{
  position:absolute;
  z-index:15;
  display:none;
  width:300px;
  top:20px;
}

/* level 3+ */
.nav ul ul ul{
  top:10px;
  left:280px;
}

/* 2nd level+ items vertically */
.nav li li{
  float: none;
}

/* when mouse over a list item, show its child nodes */
.nav li:hover ul, .nav li li:hover ul,
.nav li li li:hover ul, .nav li li li li:hover ul{
  display:block;
}

/* related to the above property, hide the child\'s child nodes :) */
.nav li:hover ul ul, .nav li:hover ul ul ul,
.nav li:hover ul ul ul ul{
  display:none;
}