[css-background] background shorthand and "/" slash syntax

[karlcow]

The CSS spec says:
cover attribute is allowed only if a position has been specified before hand.

<bg-layer> =    <bg-image> 
                     || <bg-position> [ / <bg-size> ]? 
                     || <repeat-style> 
                     || <attachment> 
                     || <box> 
                     || <box>

Currently Safari, Chrome on one side behaves differently than Firefox.
Chrome and Safari accepts this syntax and shows the image (not following the spec)

background: rgba(0,0,0,0) 0 url(…) / cover

Test: https://codepen.io/webcompat/pen/LYPLvow

• Firefox Bug 1577665
• Chrome bug 1002310
• Safari bug 201623