Edit in JSFiddle

<div class = "fallback-message">
    <p>impress.js not supported</p>
</div>
<div id = "impress">
    <div class = "step">
        Slide 1
    </div>
    <div class = "step" data-x = "1000">
        Slide 2
    </div>
</div>

<script src = "https://raw.github.com/bartaz/impress.js/master/js/impress.js"></script>
<script>impress().init();</script>

.impress-supported .fallback-message {
    display: none;
}