How to embed an autoplaying YouTube video in an iframe?

This works in Chrome but not Firefox 3.6 (warning: RickRoll video):

<iframe width="420" height="345" src="http://www.youtube.com/embed/oHg5SJYRHA0?autoplay=1" frameborder="0" allowfullscreen></iframe>

The JavaScript API for iframe embeds exists, but is still posted as an experimental feature.

UPDATE: The iframe API is now fully supported and "Creating YT.Player objects - Example 2" shows how to set "autoplay" in JavaScript.

Since April 2018, Google made some changes to the Autoplay Policy. You not only need to add the autoplay=1 as a query param, but also add allow='autoplay' as an iframe's attribute

So you will have to do something like this:

<iframe src="https://www.youtube.com/embed/VIDEO_ID?autoplay=1" allow='autoplay'></iframe>