I don't know much about CSS. I used block hiding when making the vertical CSS menu, but not the horizontal menu. I'm very confused. Can someone please give me an answer_html/css_WEB-ITnose

先把纵向菜单的导航代码发出来
--------------------------------------------------------

• 
  中国
  
  
  
  • 唐朝
  
  
  • 宋朝
  
  
  • 汉朝
  
  
  • 元朝
  
  
  • 明朝
  
  
  • 清朝
  
  
  
  


• 俄罗斯
  
  
  
  • 彼得大帝
  
  
  • 沙皇
  
  
  • 叶卡捷莉娜二世
  
  
  
  


• 马来西亚


• 美国


• 印度


• 日本


• 泰国


• 法国

----------------------纵向菜单导航的样式表如下--------------------------------
#menu ul {
margin: 0px;
padding: 0px;
list-style-type: none;
}
body {
font-family: "新宋体";
font-size: 12px;
line-height: 1.5;
}
#menu {
width: 100px;
border: 1px solid #FFFF66;
}
#menu ul li {
line-height: 26px;
background-color: #CCCCCC;
height: 26px;
border: 1px solid #FF9900;
position: relative;
}
#menu ul li a {
color: #FF0000;
text-decoration: none;
width: 100px;
}
#menu ul li a:hover {
color: #00FF00;
width: 100px;
background-color: #0000FF;
}

#menu ul li:hover  {
background-color: #99FF66;
}
#menu ul li ul {
display: none;
border: 1px solid #FFFF99;
position: absolute;
width: 100px;
left: 100px;
top: 0px;
}
#menu ul li:hover ul {
display: block;
}
a:hover {
background-color: #3300FF;
}
---------------------------------------------------------------
可以看到标红的地方用了隐藏，也就是说不划过的话就不显示，那么这个可以理解
下面再来看看横向菜单的全部代码-----------------------------------------

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--------------------------------------------------You can see the horizontal menu style There is no part hidden in . The secondary menu is hidden when the page is opened. It is strange that the vertical menu defines ul as a block, while the horizontal menu defines a as a block. Could it be that a is defined as a block? Doesn't it need to be hidden if it is defined as a block? Moreover, when I tried to define the second-level ul as hidden in the horizontal menu, there was no response when the mouse moved over the first-level menu. In other words, the second-level menu was completely hidden. Does this mean that this hiding is inherited? Please ask relevant experts to answer, thank you

Reply to the discussion (solution)

There is indeed no hiding in landscape orientation, it’s just you It's just invisible

#nav li ul {line-height:27px; list-style-type:none; text-align:left; left:-999em; width:180px; position:absolute;}

Did you see left:-999em;? It is outside the page display area by default.

#nav li:hover ul {left:auto;}

When the mouse passes over, left is restored to the default position, so it appears.

Of course, it is recommended that you use hide, because in the current situation -999 is definitely outside the page, but if your ul itself is 999 away from the left, it will appear in the upper left corner of the page

Well, thank you, I really didn’t know what -999em meant