W3Schools Tryit Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
body {
  margin: 0;
  padding: 0;
}

h2, p {
  margin: 10px;
}

ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
  background-color: #333333;
  display: flex;
}

ul li a {
  display: block;
  color: white;
  padding: 14px 16px;
  text-decoration: none;
}

ul li a:hover {
  background-color: #111111;
}

/* For viewport width 600px or less, make the menu links stack on top of each other */
@media screen and (max-width: 600px) {
  ul {flex-direction: column;}
}
</style>
</head>
<body>

<ul>
  <li><a href="#home">Home</a></li>
  <li><a href="#news">News</a></li>
  <li><a href="#contact">Contact</a></li>
  <li><a href="#about">About</a></li>
</ul>

<h2>Responsive navigation menu</h2>

<p>When the screen is less than 600px, the navigation menu will be displayed vertically instead of horizontally.</p>

<p><strong>Resize the browser window to see the effect.</strong></p>

</body>
</html>