Currently, HTML 4 and XHTML 1.0 forbid the nesting of OPTGROUPs
<!ELEMENT OPTGROUP - - (OPTION)+ -- option group -->
But state:
Note. Implementors are advised that future versions of HTML may extend
the grouping mechanism to allow for nested groups (i.e., OPTGROUP
elements may nest). This will allow authors to represent a richer
hierarchy of choices.

Is this planned for XHTML 2? Does XForms 1.0 already allow nesting of
option groups?