we're still not checking whether the url is actually for an http or https resource. The mere presence of `property: url(...)` is not enough to say this is mixed content, we need to check for `property: url(http://` or `property: url('http://` or `property: url("http://`. Right now this regexp simply checks whether there is a `url(...)` at all, irrespective of what it's actually loading (it could be a data-uri or https, in which case even with mixed content enforced, those are fine).