According to https://dom.spec.whatwg.org/#interface-domtokenlist tokenList[index] should work exactly like tokenList.item(index) but the first one always returns undefined. I've encountered some errors becasue of that.
Attached patch contains test and fix (probably wrong one but it works).
Thanks for reporting (and the patch), committed in SVN.
Please provide your name if you want it to be referenced.
I've set my name. Thanks.
Commited, thanks.
https://sourceforge.net/p/htmlunit/code/12518/