Modernizr reports that Opera Mini supports background-size, even though it doesn't.
Tested with Opera Mini 6.5.26571 on http://haz.io/.
Looks like it's false positive on quite a few properties, comparing haz.io results too Opera support docs
I've filed a bug for this with Opera, they're looking into it.
I will comment here when there is any update regarding this.
documented opera mini's false pos on background-size. fixes #396