<!DOCTYPE html>
<html>
<head>
<style>
.foo { display: flex; }
div { margin: 10px; border: 1px solid black; }
#bar { flex: 1; font-size: 18px; }
</style>
</head>
<body>
<div class="foo">
<div>All your base</div>
<div id="bar">Are belong to us</div>
</div>
</body>
</html>