Edit in JSFiddle

.border-radius {
    border-radius: 20px;
    margin: 20px;
    padding: 20px;
    background: #ddd;
    /* useful if you don't want a bg color from leaking outside the border: */
    -moz-background-clip: padding;
    -webkit-background-clip: padding-box;
    background-clip: padding-box;
}
<div class="border-radius">
    border-radius
</div>