How do I extend the container around the wrapped elements so when I put in content it extends automatically? The container is stuck at the top of the page above the header even though the header is wrapped in the container div.
HTML
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>basic</title>
    <link rel="stylesheet" type="text/css" href="style.css">
</head>
<body>
   <div id="container">
       <div id="header">
           <div class="nav_left">
               <a href="#">Home</a>
               <a href="#">About</a>
               <a href="#">Download</a>
               <a href="#">Contact</a>
           </div>
       </div>
               <div id="bigbox">
               </div>
   </div>
</body>
</html>
CSS
html{
}
body{
    margin: 0;
    padding: 0;
    background-color: grey;
}
#container{
    width: 80%;
    margin-left: 10%;
    margin-right: 10%;
}
#header{
    width:80%;
}
.nav_left{
    float:left;
    height:30px;
}
#bigbox{
    width:80%;
}
 
    