I thought long and hard about whether it was appropriate for me to comment on web application issues given my job, but I'm sure everyone will understand that this is just my personal opinion and not sales pitch.
Somewhere in Sheffield City council there has been a process failure that means you cannot make full use of the planning portal on a Windows XP machine without IE or in most browsers on the other operating systems..
The fact that they have bothered to ensure they are at least W3C-WAI AA compliant and have gone and got the Bobby badge, suggests that they take the concept of access to data by as many people as possible at least a little seriously and it is a QA issue rather than a conscious decision to exclude people.
So what is the issue?
 I couldn't be bothered to go and try it on Vista, interestingly there is specific code to fix this specific problem for KHTML browsers like Konqueror and Safari