It seems reasonable to me to walk up the folder hierarchy looking for HTML files in each folder (if only one found, use that; if more than one found, prefer index.*).  And if all else fails, if there's only one HTML file in the whole project then use that.  That's a little above and beyond the user story but it seems like a nicer heuristic.

Or we could be slightly simpler and always ignore anything other than index.* but that seems less nice.