How to include the background-cover value in the shorthand background property?

Question

According to the W3 and MDN, there needs to be a slash separating the backgound-size from the background-position:

W3C example:

p { background: url("chess.png") 40% / 10em gray  round fixed border-box; }

MDN:

This property must be specified after background-position, separated
with the “https://stackoverflow.com/” character.

Opera also has some information on the background shorthand:

http://dev.opera.com/static/dstorey/backgrounds/background-shorthand.html