HTML Reference Guide
<OPTGROUP> ... </OPTGROUP> OPTION GROUP
The OPTGROUP element allows authors to group selection list choices into a hierarchy. This is particularly helpful to non-visual user agents when large numbers of options are available to choose from. Possible rendering of this element could be a collapsible hierarchy list of OPTION element content or some presentatin conveying the hierarchy relationship of the OPTION contents. Browsers not supporting this element will "see" only a traditional flat list of OPTION elements. The LABEL attribute for this element is used to provide shorter labels for the nodes of the hierarchical menus.
disabled - This is a stand-alone attribute which indicates the element is initially non-functional.
label - This specifies a shorter alternate label for use in identifying the sub-group of options.