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I was certain there'd be /some/ kind of JS in there..! Rather impressive.

For sure! What is found at the end of the page is priceless, sophisticated humor: <script type="text/javascript">

</script>

I think it's automatically inserted by jsdo.it's backend.

Indeed. I had to audit the source believe it, but this is actually legitimate.

Massive abuse of data url's and keyframes, but it works.

In what way is it abusing data urls? Carrying small images is a pretty popular use.