<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
  <head>
  <title>z-index</title>
  <style type="text/css">
    div {
      position: absolute;
    }
  </style>
  </head>
  <body>
    
    <div style="background: #fff; opacity: 0.8; border: 6px solid red; z-index: 3; padding: 3em; top: 3em; left: 2em; width: 6em;">
    z-index: 3, order: 1
    </div>
    
    <div style="background: #ff