#progressbar {
background-color: black;
border-radius: 13px;
/* (height of inner div) / 2 + padding */
padding: 3px;
}
#progressbar>div {
background-color: orange;
width: 40%;
/* Adjust with JavaScript */
height: 20px;
border-radius: 10px;
}
<div id="progressbar">
<div></div>
</div>
Fiddle
(EDIT: Changed Syntax highlight; changed descendant to child selector)