CSS-only Rainbow Divider

Demo

hr {
    position:relative;
    height:.25em;
    border:0;
    overflow:hidden;
}
hr:before {
    content:"";
    display:block;
    position:absolute;
    top:0;
    left:0;
    height:.25em;
    width:5em;
    background:red;
    box-shadow:
        5em 0 0 0 orange,
        10em 0 0 0 yellow,
        15em 0 0 0 green,
        20em 0 0 0 blue,
        25em 0 0 0 indigo,
        30em 0 0 0 violet;
}

Source