This can be done in such a way as avoid problems for people using existing browsers. Authors would need to consider both old and new browsers when writing HTML pages. Regular windows based browsers would be able to render option groups in a variety of ways, e.g. horizontal rules between groups, slide-right menus as used by Windows 95, or tabbed dialogs. Everyone wins, including people with disabilities. The WAI-HC list has been discussing two proposals.
Authors provide labels for options that make sense when viewed with older browsers.
In the absence of the context provided by the option groups, these labels need to spell-out the full meaning of each option. For newer browsers that do show the option groups, this would make the options look rather long winded. Newer browsers use this attribute, when present, in preference to the element's content when fetching the label for each option. This proposal makes the option group structure immediately apparent in the markup, particularly so if the elements are indented to reflect the level of nesting. The primary difference between the two proposals are case where differentiation is not important, or not desirable.