According to the HTML 4.01 Specification:

If no radio button in a set sharing the same control name is initially "on", user agent behavior for choosing which control is initially "on" is undefined.
...
Since user agent behavior differs, authors should ensure that in each set of radio buttons that one is initially "on".

As to the button clearing all choices, as far as I know it would require using javascript, which is not supported by lynx.

Cheers

Antonio
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The stupider the astronaut, the easier it is to win the trip to Vega - A. Tucket