W3Schools Tryit Editor

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
* {
  box-sizing: border-box;
}

/* Style the top navigation bar */
.topnav {
  overflow: hidden;
  background-color: #333;
}

/* Style the topnav links */
.topnav a {
  float: left;
  display: block;
  color: #f2f2f2;
  text-align: center;
  padding: 14px 16px;
  text-decoration: none;
}

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

/* On screens that are 600px wide or less, make the menu links stack on top of each other instead of next to each other */
@media screen and (max-width: 600px) {
  .topnav a {
    float: none;
    width: 100%;
  }
}
</style>
</head>
<body>

<h2>Responsive navigation menu</h2>
<p>Resize the browser window to see the effect: When the screen is less than 600px, the navigation menu will be displayed vertically instead of horizontally.</p>

</body>
</html>

​x
 
<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1"><style>* {  box-sizing: border-box;}​/* Style the top navigation bar */.topnav {  overflow: hidden;  background-color: #333;}​/* Style the topnav links */.topnav a {  float: left;  display: block;  color: #f2f2f2;  text-align: center;  padding: 14px 16px;  text-decoration: none;}​/* Change color on hover */.topnav a:hover {  background-color: #ddd;  color: black;}​/* On screens that are 600px wide or less, make the menu links stack on top of each other instead of next to each other */@media screen and (max-width: 600px) {  .topnav a {    float: none;    width: 100%;  }}</style></head><body>​<h2>Responsive navigation menu</h2><p>Resize the browser window to see the effect: When the screen is less than 600px, the navigation menu will be displayed vertically instead of horizontally.</p>​