WebKit currently has: enum PageVisibilityState { PageVisibilityStateVisible, PageVisibilityStateHidden, PageVisibilityStatePrerender, PageVisibilityStatePreview }; but the current spec lists: On getting, visibilityState attribute MUST return one of the following DOMStrings or a vendor prefixed DOMString as defined in 4.5 Vendor Prefixes: hidden, visible, prerender, unloaded.
"preview" was in http://www.w3.org/TR/2011/WD-page-visibility-20110721/ but was replaced by "unloaded" in http://www.w3.org/TR/2012/CR-page-visibility-20120726/