http://dev.w3.org/csswg/cssom-view/#dom-element-scrolltop says:

"
(...)
When setting the scrollTop attribute these steps must be run:
(...)
4 - If the element is the root element invoke scroll() with zero as first argument and y as second, and, if the given value is a ScrollOptionsVertical object, the given value as the third argument.
(...)
"

This is not what browsers do, as one can see here running this sample here: http://jsfiddle.net/4bRQc/4/ . IE, Firefox, Opera 12 (pre-blink) and Safari nightly builds do not zero X when setting scrollTop on the root/documentElement node. Similarly, when setting scrollTop on body node, in quirks-mode.

ps: Blink/Opera work to match other engines, is under works.

Unless, this divergence between the spec description and web engines' behavior is still relevant at this point, I propose wording it like:
"4 - If the element is the root element invoke scroll() with *scrollX* as first argument and y as second, and, if the given value is a ScrollOptionsVertical object, the given value as the third argument."

That would also make step (4) to match step (5).