[danik.js]

<script>
Object.defineProperty(Element.prototype, 'fadeOut', {
    enumerable: false,
    value: function() {
        this.style.opacity = window.getComputedStyle(this).opacity;
        (function(){
            (this.opacity-=.05) < 0 ? this.display = "none": setTimeout(arguments.callee.bind(this), 100)}
        ).call(this.style)
    }
});
</script>
<button onclick="this.fadeOut()">Нажми меня</button>

Старые IE в пролете.