background-opacity property proposal

Hello. It would be nice to have background-opacity property.

It's like rgba() but far more useful: background-opacity property could control opacity of _entire_ background including background color _and_ image together.

For example, currently we are limited to apply CSS transitions to only background color opacity (via rgba()) which is useless in most of cases when an element have not only background-color, but also background-image assigned. background-opacity would allow to control and animate opacity of entire background:

example {background: #000 url(example.jpg); background-opacity: .5; }

Thanks.