It is not used.
It IS used. Will reuse this bug to fix the shorthand behavior in the Styles sidebar pane. For the style source: margin-left: 1px; margin-top: 1px; margin: 2px; the Styles pane displays the following contents: margin: 2px; margin: 2px;
Created attachment 143278 [details] Patch
Committed r117958: <http://trac.webkit.org/changeset/117958>