ISSUE-12: We need a "mixed" or "undefined" value for the pressed state

Added mixed state to draft 27 August 2007 http://www.w3.org/2007/08/27-pf-minutes

In the 27 August telecon we decided to add a 'mixed' value to the 'pressed' state on the 'button' role.

While this resembles the 'mixed' state in the tristate checkbox, the state transition graph is not always the same. In the case of 'alignment' controls the buttons act more like radio buttons, in that only one can be in effect at once. There is still the possibility of a 'mixed' state if the selected text spans subranges that have different alignment (left, center, right, full). But pressing one of the buttons does not clear the button, but rather sets that button and clears the competing buttons.


http://www.w3.org/2007/08/27-pf-minutes.html#item04