Empty containers not allowed

> Note: When multiple roles are specified as "Required Child Elements"  
> for a role, at least one instance of one required child element is  
> required. This specification does not require an instance of each of  
> the listed child roles. For example, a menu should have at least one  
> instance of a menuitem, menuitemcheckbox, or menuitemradio. The menu  
> role does not require one instance of each.

Why aren't empty containers allowed? Empty containers might arise from  
natural application states, and putting placeholders in containers  
just complicates things.

-- 
Henri Sivonen
hsivonen@iki.fi
http://hsivonen.iki.fi/