What exactly are the rules for avoiding the “mixed content” warning in IE due to background images?

Question

This is related to SSL and mixed content due to CSS background images but that question had no accepted answer and the one I\'m asking is a little more specific.

Under s

Answer 1

Using the fully-qualified URI will avoid the problem of IE8 and below incorrectly creating a bogus URI like about:/relative/file.png that triggers the mixed content warning. This problem was fixed in IE9.