Как вариант...
<!DOCTYPE html>
<html>
<head>
<meta http-equiv='Content-Type' content='text/html; charset=windows-1251' />
<!--
<script src='http://code.jquery.com/jquery-latest.js'></script>
<script src="https://code.angularjs.org/1.3.9/angular.min.js"></script>
<script src="https://code.angularjs.org/1.3.9/angular-route.js"></script>
-->
<style type='text/css'>
.wrapper {
display: flex;
justify-content: space-between;
flex-wrap: wrap;
border: 1px solid;
}
.content {
flex-basis: auto;
flex-grow: 1;
flex-shrink: 1;
min-width: 400px;
max-width: 800px;
height: 100px;
background-color: red;
}
.sidebar {
flex-basis: 200px;
height: 100px;
background-color: green;
}
</style>
<script type='text/javascript'>
</script>
</head>
<body>
<div class="wrapper">
<div class="content">content</div>
<div class="sidebar">sidebar</div>
</div>
</body>
</html>