optgroup
The optgroup element allows the author to group related option elements into a hierarchy. The browser may use this hierarchy to facilitate layout and presentation on a wide variety of devices.

attributes

title
Specifies a title for this element, which may be used in the presentation of this object.
id
Provides an element with a unique name within a single deck.
class
Affiliates an element with one or more classes.
xml:lang
Specifies the natural or formal language in which the document is written.