These were added in http://trac.webkit.org/changeset/50477.
However, the implementation was not completed in any WebKit port and the JS-available methods do nothing, making feature detection to fail. I'm basically reverting the r50477. If and when there will be enough interest to implement the feature (in presently-specified or perhaps revised form), it should be easy to re-revert. Chromium currently lists this as p2 feature request: http://crbug.com/11359.
For the record, FF 3.x implements the feature, while Opera 10.x implements the feature in an extended form, trying to achieve a better integration with existing web mail services (http://operawiki.info/WebMailto, http://weblog.timaltman.com/archive/2009/01/13/web-mail-integration-opera-100)
Created attachment 60914 [details]