<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
html,body{
height: 100%;
}
.box{
display: flex;
height: 100%;
background-color: pink;
}
.content{
height: 100%;
}
</style>
</head>
<body>
<div class="box">
<div class="content">11</div>
<div></div>
</div>
</body>
</html>