W3Schools Tryit Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
* {
  box-sizing: border-box;
}

.left {
  background-color: #2196F3;
  padding: 20px;
  float: left;
  width: 20%; /* The width is 20%, by default */
}

.main {
  background-color: #f1f1f1;
  padding: 20px;
  float: left;
  width: 60%; /* The width is 60%, by default */
}

.right {
  background-color: #04AA6D;
  padding: 20px;
  float: left;
  width: 20%; /* The width is 20%, by default */
}

/* Use a media query to add a break point at 800px: */
@media screen and (max-width: 800px) {
  .left, .main, .right {
    width: 100%; /* The width is 100%, when the viewport is 800px or smaller */
  }
}
</style>
</head>
<body>

<h2>Media Queries</h2>
<p>Resize the browser window.</p>

<p>Make sure you reach the breakpoint at 800px when resizing this frame.</p>

<div class="main">
  <p>Main Content</p>
</div>

<div class="right">
  <p>Right Content</p>
</div>

</body>
</html>

​x
 
<!DOCTYPE html><html><head><meta name="viewport" content="width=device-width, initial-scale=1.0"><style>* {  box-sizing: border-box;}​.left {  background-color: #2196F3;  padding: 20px;  float: left;  width: 20%; /* The width is 20%, by default */}​.main {  background-color: #f1f1f1;  padding: 20px;  float: left;  width: 60%; /* The width is 60%, by default */}​.right {  background-color: #04AA6D;  padding: 20px;  float: left;  width: 20%; /* The width is 20%, by default */}​/* Use a media query to add a break point at 800px: */@media screen and (max-width: 800px) {  .left, .main, .right {    width: 100%; /* The width is 100%, when the viewport is 800px or smaller */  }}</style></head><body>​<h2>Media Queries</h2><p>Resize the browser window.</p>​<p>Make sure you reach the breakpoint at 800px when resizing this frame.</p>