Specifying an element with a radio creates a new theme widget which is an x::w::image_button radio button widget. This is equivalent to invoking the factory's create_radio() method.

The radio element contains a theme widget or container that creates the contents of the radio button, usually a label element.

An optional appearance value provides create_checkbox()'s optional x::w::const_image_button_appearance parameter. The value of the appearance element is a type=“image_button” appearance object from the theme file.

The required group element sets the first parameter create_radio(), and gives the radio button group's name. All generate()d radio buttons with the same group name get created with the same x::w::radio_group handle.