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<optgroup> : option group
The OPTGROUP element allows authors to group choices logically. This is particularly helpful when the user must choose from a long list of options; groups of related choices are easier to grasp and remember than a single long list of options. In HTML 4, all OPTGROUP elements must be specified directly within a SELECT element (i.e., groups may not be nested).
-) name
This attribute assigns the control name.
-) size
If a SELECT element is presented as a scrolled list box, this attribute specifies the number of rows in the list that should be visible at the same time. Visual user agents are not required to present a SELECT element as a list box; they may use any other mechanism, such as a drop-down menu.
-) multiple
If set, this boolean attribute allows multiple selections. If not set, the SELECT element only permits single selections.
Attributes defined elsewhere
-) id, class (document-wide identifiers)
-) lang (language information), dir (text direction)
-) title (element title)
-) style (inline style information)
-) disabled (disabled input controls)
-) tabindex (tabbing navigation)
-) onclick, ondblclick, onmousedown, onmouseup, onmouseover, onmousemove, onmouseout, onkeypress, onkeydown, onkeyup (intrinsic events)


<select name="names_sel">
    <optgroup label="names">


For complete reference see html reference at W3C
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