body {
 width: 99%;
 margin: 2px auto;
 font-family: Arial, "MS Trebuchet", sans-serif;
 text-align: center;
}

img.center {  
  display: block; 
  margin:2px auto;
  border:1px solid #000000;
  margin-left: auto; 
  margin-right: auto;
  }
  
header, footer, section, aside, nav, article { display: block; }

header {
 width: 90%;
 margin: 4px auto;
 border: 2px solid blue;
}

nav ul li { 
display:inline;
margin: 0 20px 0 0;
padding: 1px;
}

article {
 width: 90%;
 margin: 10px auto;
 background-color: yellow;
 text-align: left;
 padding-left: 5px;
}

 article p.next-to-aside {
  width: 99%;
  }
 
aside.right {
 position: relative;
 left: 0px;
 top: -10px;
 width: 20%;
 float: right; 
 z-index: 1; 
 padding-left: 5px;
 padding-top: 5px;
 background-color: #00ff00;
 border: 2px solid #000000;
 text-align: center;
}

aside.left {
 position: relative;
 left: -5px;
 top: -10px;
 width: 20%;
 float: left; 
 z-index: 1; 
 padding-right: 5px;
 padding-top: 5px;
 background-color: #00ff00;
 border: 2px solid #000000;
 text-align: center;
}

footer {
clear: both;
width: 90%;
margin: 4px auto;
border: 2px solid blue;
font-family: serif;
}