I couldn't find any post related to this in the WP forums, so my apologies if this was discussed elsewhere.
This isn't a problem with NextGen at all (which is one of my favorite WP plugins), but with Thickbox really: thickbox.css doesn't pass W3C's CSS validation (and, yeah, that's kind of important to me... not necessarily a deal breaker for me personally, but if I build a site for someone else, I like it to be as squeaky clean as possible code-wise). From what I could glean at the Thickbox forum, fixing that is left to the user since it is not a priority for the developers.
Would it be hard for me to tweak NextGen to use instead, say, Smootbox (a Thickbox clone)? I'm not really interested in generating a jQuery vs. mootools discussion. I'd rather use the latter since Smoothbox does pass CSS validation out of the box.