CSS3 UI "icon" property

The "icon" property in CSS3 UI seems to not define the behavior when none of the 
icons in the list can be rendered by the user-agent. It would seem that defining 
this case is necessary to ensure interoperable implementation (read that as "I 
don't know what to implement, if I were to try to make this work").

Also, it is not clear what the use of the "auto" value is, since the icon that 
will be shown is completely arbitrary.  I fear that whatever icon the first UA 
to market randomly picks for this others will need to follow, creating a 
needless de-facto standard.

If there is a reason for having this value, there probably needs to be some 
description of what the default icon should designate on a conceptual level (eg 
warning icon, error icon, meaningless abstract-art icon, whatever).

-Boris