Sticky

HTML

<!DOCTYPE html>
<html lang="ru">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <header>
    <h1>Тут шапка сайта</h1>
  </header>
  <div>
    <h2>А тут не шапка сайта</h2>
  </div>
  <main>
    <p>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Unde facere dolores odio est nesciunt, velit ea fuga placeat numquam, omnis ipsum qui voluptate eum quaerat tenetur recusandae voluptates, deserunt praesentium.</p>
    <p>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Unde facere dolores odio est nesciunt, velit ea fuga placeat numquam, omnis ipsum qui voluptate eum quaerat tenetur recusandae voluptates, deserunt praesentium.</p>
    <p>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Unde facere dolores odio est nesciunt, velit ea fuga placeat numquam, omnis ipsum qui voluptate eum quaerat tenetur recusandae voluptates, deserunt praesentium.</p>
  </main>
</body>
</html>

CSS

body {
  margin: 0;
  height: 1000px;
}

h1,
h2 {
  margin: 0;
}

header {
  padding: 50px 20px;
  color: #fff;
  
  background: #90CAF9;
}

div {
  position: sticky;
  top: 0;
  padding: 20px;
  
  color: #fff;
  
  background: #EF9A9A;
}