html how to ensure that the header always on top

/* The navigation bar */
.navbar {
  overflow: hidden;
  
  background-color: #333;
  position: fixed; /* Set 
  the navbar to fixed position */
  top: 0; 
/* Position the navbar at the top of the page */
  width: 
100%; /* Full width */
}

/* Links inside the navbar */

  .navbar a {
  float: left;
  display: 
  block;
  color: #f2f2f2;
  text-align: 
  center;
  padding: 14px 16px;
  
  text-decoration: none;
}

/* Change background on mouse-over */
.navbar 
  a:hover {
  background: #ddd;
  color: 
  black;
}

/* Main 
content */
.main {
  margin-top: 30px; /* Add a top 
  margin to avoid content overlay */
} 

#header {
  position: fixed;
}

#content {
  margin-top: 100px;
}