<div id="sidebar"></div>
<div id="page"></div>
<div id="sidebar"></div>
#topbar {
width: 100%;
height: 100px;
}
#page{}
#sidebar {}
<!DOCTYPE html>
<html lang="ru-RU">
<head>
<meta charset="UTF-8">
<meta name="author" content="reskwer">
<title>Что-то на CodePen</title>
<link rel="stylesheet" href="style.css">
</head>
<body>
<div class="content">
<div id="topbar">Высота 100px</div>
<div id="page">Высота 50% от 100%-100px</div>
<div id="sidebar">Высота 50% от 100%-100px</div>
</div>
</body>
</html>
html,body{
width: 100%;
height: 100%;
padding: 0;
margin: 0;
text-align: center;
}
.content{
width: 100%;
height: 100%;
position: relative;
}
#topbar {
width: 100%;
height: 100px;
background: #eee;
z-index: 3;
position: absolute;
top: 0;
box-sizing: border-box;
padding-top: 21px;
font-size: 200%;
}
#page{
width: 100%;
height: 50%;
background: #aaa;
margin-top: -50px;
z-index: 2;
position: absolute;
top: 100px;
box-sizing: border-box;
padding-top: 11%;;
font-size: 200%;
}
#sidebar {
width: 100%;
height: 50%;
background: #666;
z-index: 1;
position: absolute;
bottom: 0;
box-sizing: border-box;
padding-top: 11%;
font-size: 200%;
}
<body>
<div class="content">
<div id="topbar">Высота 100px</div>
<div id="page">Высота 100% от 100%-100px</div>
<div id="sidebar">Высота 100% от 100%-100px</div>
</div>
</body>
html,body{
width: 100%;
height: 100%;
padding: 0;
margin: 0;
text-align: center;
}
.content{
width: 100%;
height: 100%;
position: relative;
}
#topbar {
width: 100%;
height: 100px;
background: #eee;
z-index: 3;
position: absolute;
top: 0;
box-sizing: border-box;
padding-top: 21px;
font-size: 200%;
}
#sidebar{
width: 200px;
height: 100%;
background: #aaa;
z-index: 2;
position: absolute;
box-sizing: border-box;
padding-top: 100px;
font-size: 200%;
}
#page {
width: 100%;
height: 100%;
background: #666;
z-index: 1;
position: absolute;
box-sizing: border-box;
padding-top: 100px;
padding-left:200px;
font-size: 200%;
}