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I'm curious as to why only XKCD comics are eligible for oneboxing in chat. XKCD is not the only webcomic out there, but as far as I know no other comics onebox by default.
There's not much to it. :) XKCD provides comic data in JSON that we can easily consume, it's a comic that gets quoted a bunch in chat, and at one point a developer cared enough to write-up the parser for the onebox.