No, overflow-x and y are not CSS3 selectors. They are simply properties, similar to overflow but with direction restrictions.
From http://reference.sitepoint.com/css/overflow
The CSS3 overflow-x and overflow-y
properties are partially supported in
Internet Explorer for windows versions
5 and 6 and fully supported in
Internet Explorer versions 7 & 8.
Safari 3+, Chrome 2+, Firefox 2+ and
Opera 9.5+ all support these
properties.