Nevermind everyone, I managed to solve the problem by adding an additional div between the wrapper and box.
CSS
#wrapper {
position: absolute;
}
#middle {
border-radius: 100px;
overflow: hidden;
}
#box {
width: 300px; height: 300px;
background-color: #cde;
}
HTML
<div id="wrapper">
<div id="middle">
<div id="box"></div>
</div>
</div>
Thanks everyone who helped!