Proper support for optgroup disabled in 4

Expected results: options D and F should be not focusable and not selectable.

Example taken from the excellent testcase: Robin Lionheart HTML 4 Conformance test: Forms

Disabled controls do not receive focus; disabled controls are skipped in tabbing navigation; disabled controls cannot be successful.
coming from HTML 4.01, section 17.12.1 Disabled controls

if an optgroup is disabled, [then] all option children are disabled too, from Vincent Ricard in KDE bug report 76489

Internet Explorer 8 final release, Firefox 3.0.11, Opera 9.64 and Safari 4.0 pass this test.

This bug has been reported at KDE Bug Tracking System as bug 197452.