<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>This text is the title of the document</title>
  <style>
    header {
      height: 100px;
      background: #CCCCCC;
    }
    header:hover h1 {
      display: none;
    }
    </style>
</head>
<body>
  <header>
    <h1>Big Title</h1>
    <h2>Small Title</h2>
  </header>
</body>
</html>