Pin a flexible-height header to the top of the viewport in desktop browsers above the scrollable content.
html,
body {
height: 100%;
margin: 0;
}
.wrapper {
height: 100%;
display: flex;
flex-direction: column;
}
.content {
flex: 1;
overflow: auto;
}
<!doctype html>
<html><!-- 100% height -->
<head>
<!-- ... -->
</head>
<body><!-- 100% height -->
<div class="wrapper">
<header>Sticky Header</header>
<main class="content">
Content with Lorem ipsum dolor...
</main>
</div>
<body>
</html>
Live preview: https://jsfiddle.net/cichy380/aq95g38k/