Specification: http://www.whatwg.org/specs/web-apps/current-work/multipage/common-dom-interfaces.html#radionodelist

In step 2 of the specification for getting the "value" attribute, the spec says:

If element is null, or if it is an element with no value attribute, return the empty string.

However, as specified in section 4.10.5 (http://www.whatwg.org/specs/web-apps/current-work/multipage/the-input-element.html), input elements of type radio have value mode "default/on", such that their .value is supposed to be "on" if undefined (http://www.whatwg.org/specs/web-apps/current-work/multipage/common-input-element-attributes.html#dom-input-value-default-on). As such, it would make sense to add separate steps 2a and 2b (or whatever):

2a. If element is null, return the empty string.
2b. If the element has no value attribute, return "on".

Indeed, some of the tests at https://github.com/w3c/web-platform-tests/blob/master/html/infrastructure/common-dom-interfaces/collections/radionodelist.html currently fail on Chrome (and in my proposed Gecko patch, for https://bugzilla.mozilla.org/show_bug.cgi?id=779723) due to this issue. I submitted a pull request for the test: https://github.com/w3c/web-platform-tests/pull/1108.