Code is speech. Open source projects often have a subtext of affecting a political or social change either within a software community, or sometimes beyond. In this particular case, my reading of the text is that Zed wishes to create a baseline of ridicule for those who would prefer to alter software to support extremely old browsers. It's a fair position, and while he may be able to make that point without vulgar language he may also be effectively speaking to his audience with his crude humor.
As merijnv noted above, if you feel so strongly about language you're free to not use his project. He is free to speak and code as he wishes and you are free to support what you wish. This freedom of expression is extremely valuable, not because the word "fuck" has any particular intrinsic value, but because it's a culture of permissiveness which enables people to speak their minds and attempt to affect change even if they're clumsy and crude with language. Because it allows comedy -- and everyone's perception of comedic value is different.