The relList IDL attribute defined for <a> and <area> has not been been implemented (Firefox, Chrome, IE all say it’s undefined). There is no appropriate definition for it, as it just described as follows: “The IDL attribute relList must reflect the rel content attribute.” The meaning of “reflect” is defined in terms of using the same value, but the rel attribute is a string, whereas relList is declared to be a DOMTokenList object.

The intent is probably that relList should have the rel attribute value as parsed to a list, but this is not specified. This feature would be some usefulness, but since it has not been implemented and since the parsing a rel attribute value can be handled rather easily, relList should be removed at least for now.