Seattle fans are very passive and polite. I have been to four games in Seattle (Kingdom, Huskie Stadium, and the current one). As long as you are not an a-hole you will be fine.
There isn't much of a tailgate in Seatlle because of the limited space where it is located. The baseball stadium has a tailgate party across the street in the outfield terrace. Beer, food, and tv's showing the early games. Plus you get to see the inside of the baseball park for free.
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