<!DOCTYPE html>
<html>
<head>
[...]
</head>
<body>
<div id="main">
<div class="header">
[Header content]
</div>
<div class="content">
[IFRAME]
</div>
</div>
</body>
</html>
html, body {
height:100%;
}
#main {
height: 100%;
max-width: 1260px;
margin: 0 auto;
min-width: 280px;
display: flex;
flex-direction: column;
flex-wrap: nowrap;
justify-content: space-between;
align-content: stretch;
align-items: stretch;
}
.content {
flex: 1 1 auto;
}
#your-iframe {
display:block;
width: 100%;
height:100%;
border: none;
}