<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
html, body {
     margin: 0;
     width: 100%;
     height: 100%;
}

.tall {
    height: 4000px;
}

header {
     position: fixed;
     top: 0;
     left: 0;
     width: 100%;
     height: 100px;
     background: tomato;
}
</style>
</head>
<body>
    <header></header>
    <p>Hello world</p>
    <div class="tall"></div>
</body>
</html>