Before we begin, I'm sorry if this is posted somewhere inappropriately.
When I first joined Stack Exchange (via Arqade) I was given the option of using Steam to sign in. When I attempted to sign in to another Stack Exchange website (specifically The Board/Card Game site) I was not given the option to log in using Steam; my other credentials (Username/Password) didn't seem to accepted either, but that's probably not important to this question.
My point is, could a log-in via Steam "button" be used with the other Stack Exchange websites, or would it be too far a stretch? (It would at least be acceptable on The Board/Card Game site, I should think?)