HTMLIsIndexElement is not implemented. Evident from the failure in the following IETC test: http://samples.msdn.microsoft.com/ietestcenter/dominheritance/Prototype_HTMLIsIndexElement.htm
HTMLIsIndexElement is removed one year ago - https://trac.webkit.org/changeset/106373. I suppose its no more supported.
I have little idea why IE still supports it. http://www.w3.org/wiki/HTML/Elements/isindex Closing this as a WONTFIX. Thanks for bringing it up.