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Type: one of the values below
Can be used to create checkable menuitems or for radio button menuitems.
The menuitem is checked. The checked attribute determines whether the menuitem is checked or not.
The menuitem becomes part of a radio group. Other menuitems that have the same value for their name attributes are part of the same radio group. Only one menuitem in the same radio group can be checked at once.

More information on adding checkmarks to menus in the XUL tutorial