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 some circumstances when accessing an HTTPS website, IE will throw the “mixed content” warning if an element is given a style with a background image. I found one forum reference that said the warning can be avoided if you put the reference in a stylesheet, for example

#someElement a {
    width:11px;
    height:11px;
    display:block;
    overflow:hidden;
    background:url(../images/sprites_list.png) no-repeat;
    cursor:hand;
    cursor:pointer;
    background-position:0px -72px;
}

but not if you try to create the element inline, a la

$('#someElement').append("<a title='something' style='background: url(../images/sprites_list.png) no-repeat; ... // etc

and indeed, this works for me. I’ve seen others that say you have to use an absolute https URL to refer to the image, rather than a relative one.

What is the real story here? Is there some “official” explanation or at least a reference to what the rules are? Or failing that, is there a standard set of guidelines which if followed makes it extremely unlikely to trigger the warning?

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.