A disabled option-group:
The disabled attribute is supported by all major browsers.
Note: The disabled attribute of the <optgroup> tag is not supported in Internet Explorer 7 and earlier versions.
The disabled attribute is a boolean attribute.
When present, it specifies that an option-group should be disabled.
A disabled option-group is unusable and un-clickable.
In XHTML, attribute minimization is forbidden, and the disabled attribute must be defined as <optgroup disabled="disabled">.
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