Je sais que je suis proche de la solution, mais je n'arrive pas à comprendre comment à introduire le décalage. Dois-je utiliser une chaine ou un nombre ?

Voici ma page, elle fonctionne. Mais quand j'utilise le menu, mes titres se retrouvent sous le menu.

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<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<script>
function jumpto(h){
    var top = document.getElementById(h).offsetTop; //Getting Y of target e
    window.scrollTo(0, top);                        //Go there directly or some transition
}
</script>
<style>
body {margin:0;}
 
.navbar {
  overflow: hidden;
  background-color: #333;
  position: fixed;
  top: 0;
  width: 100%;
}
 
.navbar a {
  float: left;
  display: block;
  color: #f2f2f2;
  text-align: center;
  padding: 14px 16px;
  text-decoration: none;
  font-size: 17px;
}
 
.navbar a:hover {
  background: #ddd;
  color: black;
}
 
.main {
  padding: 16px;
  margin-top: 30px;
  height: 1500px; /* Used in this example to enable scrolling */
}
</style>
 
 
 
</head>
 
 
<body>
 
<div class="navbar">
        <a onclick="jumpto('lnk_01');">section 1</a>
        <a onclick="jumpto('lnk_02');">section 2</a>
        <a onclick="jumpto('lnk_03');">section 3</a>
        <a onclick="jumpto('lnk_04');">section 4</a>
 
</div>
 
<div class="main">
        <div class="titlepage">
            <h1 id="lnk_01">section 1</h1>
        </div>
        <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged.
		</p>
 
        <div class="titlepage">
            <h1 id="lnk_02">section 2</h1>
        </div>
        <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged.
		</p>
 
		        <div class="titlepage">
            <h1 id="lnk_03">section 3</h1>
        </div>
        <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged.
		</p>
 
		        <div class="titlepage">
            <h1 id="lnk_04">section 4</h1>
        </div>
        <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged.
		</p>
 
</div>
 
</body>
</html>