As far as I know, RFC 6648 does not make experimental NIDs (as defined by RFC 3406) invalid. The draft for the next RFC about URNs (currently) has, following the recommendations from RFC 6648, removed experimental NIDs (making URNs that use x- invalid). My gut says that the x- prefix...
javascript,scroll,web-standards
The W3 actually lists both methods in their spec of the Window interface: http://www.w3.org/TR/cssom-view/#extensions-to-the-window-interface You can safely use either one....