1863: no forward passing allowed at all
1866: at least three players have to be between player and goal
1907: can't be offside in your own half
1925: at least two players have to be between player and goal
1990: level counts as onside
2005: arms/hands can't be offside
200something: 'interfering with play'

Essentially I think as tactics have become more and more defensive as time has gone on, the rule has been changed entirely in one direction (i.e. to result in fewer offsides and more goals) as an attempted counterbalance. However, because of the nature of it, the only simple change you could make in this direction post-1990 was to get rid of it, so they have ended up going for the ever decreasing circles route of becoming more and more subjective about when you can and can't be offside, with the result being the current mess (exacerbated by video review).