Use clearfix in CSS
HTML
<!DOCTYPE html>
<html lang="en">
<head>
<title></title>
<meta charset="UTF-8">
<link rel="stylesheet" href="css/reset.css">
<link href="css/style2_1.css" rel="stylesheet">
</head>
<body>
<header class="header"></header>
<div class="wrapper clearfix">
<main class="main">aaa</main>
<div class="sidemenu">bbb</div>
</div>
<footer class="footer">
</footer>
</body>
</html>
CSS
.clearfix::after{
content: '';
display: block;
clear: both;
}
