-webkit-background-size has behavioural differences to background-size #28

-webkit-background-size is listed as a simple alias for background-size when it has different syntactic behaviour.
-webkit-background-size: 10px is equivalent to background-size: 10px 10px.
background-size: 10px is equivalent to background-size: 10px auto (see https://drafts.csswg.org/css-backgrounds-3/#bg-size).