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.

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Answer 1

I think the reason you're getting different results is not because the one method is "safer," but because the offending URL isn't present in the base document when IE loads it. I expect you'll get the warning if you were to place that A directly in the document instead of scripting it in after the page has loaded.

If I'm right in my diagnosis, this would mean there's no as-yet-undocumented quirk to the rules about mixed content.

Also: protocol-relative URLs are awesome. Just in general.