为什么CSS不允许我嵌套选择器块？

-2

观察：

<!DOCTYPE html> 
<html> 
<head> 
<script type="text/javascript" src="javascript/class-lib.js"></script> 
<script type="text/javascript" src="javascript/script.js"></script> 
<link rel="stylesheet" type="text/css" href="css/style.css" /> 
</head> 
<body> 
<div id="wrapper"> 
    <ul id="nav"> 
     <li><a href="#">Home</a></li> 
     <li><a href="#" class="selected">Parent 02</a> 
      <ul> 
       <li><a href="#">Item 01</a></li> 
       <li><a href="#" class="selected">Item 02</a></li> 
       <li><a href="#">Item 03</a></li> 
      </ul> 
      <div class="clear"></div> <!--".clear" div is nested within the .selected class, outside of the <ul>. Does this provide a buffer??? --> 
     </li> 
     <li><a href="#">Parent 03</a> 
     <ul> 
      <li><a name="child" href="#">Child 04</a> 
       <ul> 
        <li><a href="#">Item 01</a></li> 
        <li><a href="#">Item 02</a></li> 
        <li><a href="#">Item 03</a></li> 
       </ul> 
      </li> 
      <li><a href="#">Item 05</a></li> 
      <li><a href="#">Item 06</a></li> 
      <li><a href="#">Item 07</a></li> 
     </ul>   
      <div class="clear"></div> 
     </li> 
     <li><a href="#">Parent 04</a></li> 
    </ul> 
    <div class="clear"></div> 
</div> 
</body> 
</html>>

CSS：

#nav li ul li a:hover{ 

       #nav li ul li ul li a{ 
         visibility:visible; /*<-- the only reason why I did that was to see if something like this would actually work. It doesn't. I gotta say I'm really not a fan of this language. While I'm sure there were reasons for not implementing this kind of method and design/scripting pattern, it seems like there are just as well plenty reasons TO implement it. */ 
       } 
     } 

     #nav li ul li ul{ 
     display:block; 
     list-style:none; 
     } 

     #nav li ul li ul li{ 
     float:right; 
     clear:both; 
     width:50px; 
     height:100px; 
     background:#000; 
     } 

     #nav li ul li ul li a{ 
     visibility:hidden; 
     color:#fff; 
     }

为什么我这样做是为了看看是否像这样的实际工作的唯一原因。它没有。我得说我真的不是这种语言的粉丝。虽然我确信没有实现这种方法和设计/脚本模式的理由，但似乎有足够的理由来实现它。