I had issues with shadow box not validating with a XHTML1.1 doctype when using the gallery feature. To solve the problem I modified the "shadowbox.js" file where it matches teh contens of the `rel` attribute in the `a` tag.
line 462: gallery: /^(light|shadow)box__(.*?)__/i, // rel attribute format for gallery link PL: double underscore for XHTML validation
The markup is then changed from,
There will probably still be issues using any of the other options within the `rel` attribute in regards to the XHTML1.1 validation