See: http://www.w3schools.com/tags/att_input_checked.asp
Mulitple radio buttons having the same name will act as a group.
See: http://www.w3schools.com/jsref/dom_obj_radio.asp
In arc, the requested attribute will reflect the currently selected radio button for the group.
Also, note that arc's gentag doesn't handle the 'checked' attribute. So you you'll need to add:
(attribute input checked opstring)
Then change your code:
(def radio (n v (o c nil)) (gentag input type 'radio name n value v checked c))
arc> (radio "rdgp" 1) <input type=radio name="rdgp" value="1"> arc> (radio "rdgp" 2 "checked") <input type=radio name="rdgp" value="2" checked="checked">
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(def radio (n v) (gentag input type 'radio name n value v))