@media screen and (max-width: 320px){
.left{
float: none;
width: auto;
margin-right: 0;
margin-bottom: 20px;
background-color: #333366;
}
}
<!DOCTYPE html>
<html>
<head>
<title></title>
<style>
.left{
float: left;
width: 100px;
margin-right: 20px;
}
.right{
overflow: hidden;
background-color: #006699;
}
@media screen and (max-width: 320px){
.left{
float: none;
width: auto;
margin-right: 0;
margin-bottom: 20px;
background-color: #333366;
}
}
</style>
</head>
<body>
<div class="parent">
<div class="left">
<p>left</p>
</div>
<div class="right">
<p>right</p>
<p>right</p>
</div>
</div>
</body>
</html>