Will, there are several areas within Wikipedia or at least the English language version where humour exists and is tolerated by most.

Most obviously humour, especially of the self deprecating variety, is welcome on user pages and in usernames.

April fools day accounts for two more of them. The collaborative and premeditated one is that on that special day the main page of Wikipedia will be full of stories that look like spoofs but are as real as the museum of bad art. At this time of the year people are probably already discussing the possible featured article for that day. My hope is that some year it will be Heavy metal umlaut. One of the beauties of that process is it is the day you are most likely to have a featured article that is actually crowd sourced rather than largely the work of one individual.

Also on April fools day there are usually an assortment of humorous AFDs MFDs, RFAs and on one occasion even a request for signatureship. As long as people keep their antics out of article space there is a long tradition of this, and of criticising it for lack of originality. There are of course only so many ways in which one can argue for the article on the Earth to be deleted.

But while it is tolerated by most, there are people who don't like to see humour in the project. I suspect that some see it as a slippery slope to vandalism. Certainly there is the common excuse among vandals that they were only having a joke. Latitude is obviously cut for people who are primarily here to further the project, but editors who are solely or primarily involved in "humour" will soon find their humour at MFD.

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