Try this (but note that there will be a 5% not covered: 10% + 80% + 5% = 95%):
CSS:
.wrapper_positioner{
margin:0 auto;
width:70%;
}
.wrapper{
position:absolute;
width:70%;
height: 100%;
}
.header {
border:2px solid yellow;
margin:0 0;
height:10%;
}
.footer {
border:2px solid blue;
margin:0 0;
height:5%;
min-height: 35px;
}
.content {
border:2px solid green;
margin:0 0;
padding:0;
height: 80%;
}
html, body{
width:100%;
height: 100%;
height: auto;
margin:0px;
}
HTML:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<title> Blog </title>
</head>
<body>
<div class="wrapper_positioner">
<div class="wrapper">
<div class="header"></div>
<div class="content">
<p>*insert big text here*</p>
</div>
<div class="footer"></div>
</div>
</div>
</body>
</html>