Does a radio button have to be in a form to work?No, but if you want them to work, then yes 
They can still function like radio buttons if they aren't in a form, but if you want to pass their values onto other pages through GET/POST, they're gonna have to be in the same form that is submitted 
A lot of things will "work" and not be valid. <input type="radio"...> has to be in a form to be valid.From the HTML 4.01 DTD:
<!ENTITY % fontstyle "TT | I | B | BIG | SMALL">
<!ENTITY % phrase "EM | STRONG | DFN | CODE | SAMP | KBD | VAR | CITE | ABBR | ACRONYM" >
<!ENTITY % special "A | IMG | OBJECT | BR | SCRIPT | MAP | Q | SUB | SUP | SPAN | BDO">
<!ENTITY % formctrl "INPUT | SELECT | TEXTAREA | LABEL | BUTTON">
<!-- %inline; covers inline or "text-level" elements -->
<!ENTITY % inline "#PCDATA | %fontstyle; | %phrase; | %special; | %formctrl;">Which is to say that radio buttons, like all form controls, are inline elements. Which means they can occur where ever any other inline element may occur. If you are using some form control with a client side script and no FORM will be submitted, then you do not need a FORM element.
They can still function like radio buttons if they aren't in a form, but if you want to pass their values onto other pages through GET/POST, they're gonna have to be in the same form that is submitted A lot of things will "work" and not be valid. <input type="radio"...> has to be in a form to be valid.From the HTML 4.01 DTD:<!ENTITY % fontstyle "TT | I | B | BIG | SMALL">
<!ENTITY % phrase "EM | STRONG | DFN | CODE | SAMP | KBD | VAR | CITE | ABBR | ACRONYM" >
<!ENTITY % special "A | IMG | OBJECT | BR | SCRIPT | MAP | Q | SUB | SUP | SPAN | BDO">
<!ENTITY % formctrl "INPUT | SELECT | TEXTAREA | LABEL | BUTTON">
<!-- %inline; covers inline or "text-level" elements -->
<!ENTITY % inline "#PCDATA | %fontstyle; | %phrase; | %special; | %formctrl;">Which is to say that radio buttons, like all form controls, are inline elements. Which means they can occur where ever any other inline element may occur. If you are using some form control with a client side script and no FORM will be submitted, then you do not need a FORM element.