OK, there's obviously something to be said for looking at things on a wide variety of platforms. Just looked at the HTML verion of FF XI in Firefox on the Linux box, discovered that one image wasn't appearing. Looked at it in the directory, couldn't see anything wrong, started panicking in case there was some really obscure bug... then realised I'd capitalised the damned file name, and of course it wasn't capitalised in the HTML. Windows didn't notice. The iBook didn't notice. But if I'd put that file onto a web site nobody would have been able to see it until I fixed the problem.
This obviously need to become one of my standard checks from now on.