HTML5 should support tri-state checkboxes

Hi! The following has been added to the wiki at http://www.w3.org/WAI/UA/work/wiki/HTML5_review_by_UAWG_notes#Tri-State_Checkboxes:


    Tri-State Checkboxe

Why is there no true support for tri-state checkboxes (those with states of checked, unchecked, and mixed/undefined)? Not providing this means that authors and developers will still have to implement custom controls and behaviors for an extremely common feature.

*Use case:* Aidan uses speech recognition for input. When he views an interactive web page or web-based application that uses standard HTML5 controls, his speech recognition program can let him control them using standardized commands, such as checking or unchecking recognized checkboxes by saying "Check italic" or "Uncheck bold". However, when it encounters custom controls or controls with nonstandard behaviors, he has to resort to saying actual keystrokes, such as "Press tab. Press tab. Press tab. Press space." He uses a web-based text editor that provides a tri-state check box that is checked with the entire selection is italicized, unchecked when the entire selection is not italicized, and in a third, "mixed" state when only part of the selection is italicized. In one scenario, he accidentally checks the Italics check box then realizes he wants to change it back to the "mixed" state. Let's say it's implemented as an HTML5 input element with type=checkbox, but with 
scripting to handle the tristate behavior (perhaps as described in Shams' Blog: Tri-State Checkbox using Javascript - http://shamsmi.blogspot.com/2008/12/tri-state-checkbox-using-javascript.html); in this case the keyboard UI is not standardized, so the speech recognition utility cannot provide a corresponding voice command. In a second scenario, it's implemented as an entirely custom control.

*Use case:* Nadia uses a screen reader with the same web-based text editor that provides a tri-state checkbox for indicating and adjusting italics. Regardless of whether the author used an HTML5 input element with type=checkbox or if they used an entirely custom control, the screen reader has no way of determining which state it's really in, and so can't convey this to Nadia using speech.

*Use case:* Ryan, a keyboard user, is using the same web-based text editor that provides a tri-state checkbox for indicating and adjusting italics. Unfortunately, because each web site or web-based app has to implement its tri-state checkbox itself, they often implement entirely different keyboard UI, and so when Ryan comes to one he cannot easily figure out how to use it with the keyboard.

*Recommendation:* HTML5 should support tri-state check boxes and menu items so that user agents can provide standardized user interface for them and so assistive technology can provide alternative input and output for them.