[css-backgrounds-3] Computed value of background-image wrong #8604

The computed value of the background-image property is currently defined as "computed <image>".
Though the value is actually a list of <bg-image>s, i.e. a list of <image> values and none keywords.

So it looks like this needs to be fixed and say something like

"list, each item either the keyword none or an <image>"

Sebastian