Evidence will be available by virtue of it being posted on the internet. During the consultation period it was unearthed that, even if someone posts something innapropriate and then subsequently deletes it, that the information remains there indefinitly to be used as evidence.
Some people employ the use of 'banter', where insults are traded between people.
Whilst it's not really my sense of humour, I understand that to some people it is. Where two people do this and nobody is offended, this is unlikely to constitute an offence.
If, however, the recipient made it clear that they did not appreciate this form of humour and it continued regardless, then it probably would constitute an offence.
I expect it to follow the social model of disability discrimination, where the test of whether something is discriminatory or offensive lies in what the recipient perceives it to be.
In essence, even if you genuinly say something meant as a joke, if the recipient perceives it to be offensive as opposed to humourous, then it is not a joke.
As an example, how do you know that someone on a forum, chatroom, social networking site isn't, for example, autistic or on suicide watch?
The short answer is that you don't, so it's always more prudent to treat others with respect, dignity and courtesy at all times