Centrage vertical absolute et before

HTML

<a href="">Bonjour :)</a>

CSS

a {
    position: relative;
    display: inline-block;
    width: 150px;
    height: 150px;
    /*text-indent: -999px;*/
    outline: 1px solid red;
}

a:before {
    content: "TEST";
    position: absolute;
    top: 50%;
    right: 0;
    left: 0;

    line-height: 1;
    margin-top: -0.5em;

    background: pink;
    text-indent: 0;
    color: black;
    text-align: center;
}

JavaScript

/* http://forum.alsacreations.com/topic-4-70250-1-Centrer-verticalement-un-before-en-absolute.html */