[cssom] preferred order of shorthand props

I was curious what the reasoning behind this statement in step #3 for preferred order:

     # Move all items in shorthands that begin with "-" (U+002D) but do not begin with "-webkit-" last in the list, retaining their relative order.

What is the purpose of singling out webkit in this case and ordering it differently from other shorthands? I tried searching the list but couldn't find anything specific as to the reasoning for this.

Thanks,
Greg