Why can't radio buttons be "readonly"?

Radio buttons would only need to be read-only if there are other options. If you don't have any other options, a checked radio button cannot be unchecked. If you have other options, you can prevent the user from changing the value merely by disabling the other options:

<input type="radio" name="foo" value="Y" checked>
<input type="radio" name="foo" value="N" disabled>

Fiddle: http://jsfiddle.net/qqVGu/

I've faked readonly on a radio button by disabling only the un-checked radio buttons. It keeps the user from selecting a different value, and the checked value will always post on submit.

Using jQuery to make readonly:

$(':radio:not(:checked)').attr('disabled', true);

This approach also worked for making a select list readonly, except that you'll need to disable each un-selected option.

This is the trick you can go with.

<input type="radio" name="name" onclick="this.checked = false;" />