vertical margin collapse

by fangunxs

HTML

<div id="outer">
       <div id="inner"></div>
     </div>

CSS

html, body {
   margin: 0;
   padding: 0;
 }
 
 #outer {
   margin: 5em;
   border: 1px solid #f00;
 }
 
 #inner {
   width: 6em;
   height: 4em;
   background-color: #999;
    /*vertical margin collapse study*/
    margin: 4em;
    border: 1px solid yellow;
 }