The CSS 2.1 spec was last updated in 2011 and does not include any mention of this new stacking context behavior (which was only proposed on the CSS mailing list in May 2012 and has not yet made it into any spec).
No, they changed it to no longer comply with that spec, actually. Please do read the spec, not assume it says what the "html5rocks" marketing folks want it to say.
Comments
Those are all webkit based browsers, so it seems normal that they'd all behave the same.
Well they did not. Until Chrome changed the behavior to comply with http://www.w3.org/TR/CSS21/zindex.html
The CSS 2.1 spec was last updated in 2011 and does not include any mention of this new stacking context behavior (which was only proposed on the CSS mailing list in May 2012 and has not yet made it into any spec).
No, they changed it to no longer comply with that spec, actually. Please do read the spec, not assume it says what the "html5rocks" marketing folks want it to say.