I searched the forums and found that it's a known issue.
Using the rel="lightbox[group]" wont validate as the [ ] fail in the rel tag. Using the entities doesn't work either. I really have to get it to work on xhtml 1.1 and was wondering if there was any new ideas or updates on how to get that working?
This is what I found: Change the brackets to, for example, dashes or any other char in the funcion I'll quote in prototype.js! The final result would be something like this: