:before/:after

by redky

HTML

<!--
http://nicolasgallagher.com/demo/multiple-backgrounds-and-borders-with-css2/borders.html
-->

<!--
http://nicolasgallagher.com/demo/multiple-backgrounds-and-borders-with-css2/backgrounds.html
-->
<div class="box"></div>

CSS

.box {
    margin: 0 auto;
    position: relative;
    width: 100px;
    height: 100px;
    background: #fee;
    border: 5px solid red;
    padding: 10px;
}

.box:after {
    content: '';
    position: absolute;
    top: 5px;
    right: 5px;
    bottom: 5px;
    left: 5px;
    border: 3px solid green;
}

.box:before {
    content: '';
    position: absolute;
    top: 15px;
    right: 15px;
    bottom: 15px;
    left: 15px;
    border: 2px solid blue;
}