cant find the problem on making the article felxible.
this is the script:

<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title>Drift team LOODS 44 Home</title>
<link rel="stylesheet" href="main.css">
</head>
<body id="big_wrapper">


<header id="top_header">
<img src="http://i58.tinypic.com/2m2x1ko.jpg" border="0" alt="Drift dag bij TTT circuit">
</header>

<nav id="navi">
<ul>
<li>Home</li>
<li>Biografie</li>
<li>Werkplaats</li>
<li>Facebook</li>
<ul>

</nav>

<div id="new_div">

<section id="main_section">
<article>
<header>
<hgroup>
<h1>Title of article</h1>
<h2>Subitle of article</h2>
</hgroup>
</header>
<p>the article itself</p>
<footer>
<p>-Loods 44</p>
</footer>
</article>
</section>


<aside id="side_news">
<h4>News</h4>
5 april DRIFTMANIA!
</aside>

</div>

<footer id="the_footer">
Copyright Gijs Bomers 2015
</footer>
</div>

</body>
</html>

-----------------------------------------------------------------------------------------------------------


*{
margin: 0px;
padding: 0px;
}
h1{
front: bold 20px Tahoma;
}
h2{
front: bold 14px Tahoma;
}
header, section, footer, aside, nav, article, hgroup{
display:block;
}

body{
 text-align:center;
 width:100%; 
 display:-webkit-box;
 -webkit-box-pack: center;
}
#big_wrapper{
border: 1px solid black;
width: 800px;
margin: 10px auto;
text-align:left;
max-width: 1000px;
-webkit-box-orient: vertical;
-webkit-box-flex:1;
}

#navi{
background: lightgrey;
border-bottom: 1px solid red;
border-top: 3px solid red;
padding: 10px;
margin-top: -4px;
}

#navi li{
background: red;
display:inline-block;
width: 80px;
border: 1px solid black;
border-radius: 5px;
text-align:center;
list-style:none;
padding: 10px;
font: bold 14px Tahoma
}

#main_section{
float: left;
width: 560px;
margin: 25px; /*620px, 180 left*/
-webkit-box-flex:1;
}

#side_news{
float:left;
width: 110px;
margin: 25px 0px;
padding: 30px;
background: lightgrey;
border: 1px solid red;
-webkit-box-flex: 0;
}

#the_footer{
clear:both;
text-align:center;
padding:20px;
border-top: 2px solid black;
font: 10px Tahoma
}

article {
background: lightgrey;
border: 1px solid red;
padding: 20px;
margin-bottom: 15px;
}

article footer{
text-align:right;
}