Slate supports the full richness of HTML with its online decision letters. To add some additional multimedia flair, you can include the code, as highlighted below, into your decision letter to autoplay a Youtube video when the letter is loaded.
<html>
<body>
<p>Dear {{preferred}},</p>
<p>Congratulations!</p>
<div id="video_popup" style="background: black; display: none; position: fixed; top: 0; right: 0; bottom: 0; left: 0; z-index: 999;">
<div style="position: absolute; top: 0; left: 0; bottom: 50px; right: 0;">
<iframe frameborder="0" height="100%" src="//www.youtube-nocookie.com/embed/whmyr9OX8qM?rel=0&autoplay=1&showinfo=0" width="100%"></iframe>
</div>
<div style="position: absolute; bottom: 15px; left: 0; right: 0; text-align: center;">
<button class="default" onclick="$(document.body).css('overflow', ''); $('#video_popup').remove();" type="button">Skip/Close</button>
</div>
</div>
<script type="text/javascript">$(document.body).css('overflow', 'hidden'); $('#video_popup').show();</script>
</body>
</html>