How to make a transparent canvas without error?

I'm trying to get the background of a html5-starfield transparent.But why error?

This with background color black http://www.thediscopalace.com/testing.php

And this if background transparent http://www.thediscopalace.com/testing1.php

I change context.fillStyle='rgb(36,14,118)'; = BLACK to context.fillStyle='rgba(36,14,118, 0)'; = TRANSPARENT