I always thought it was the end line. The reason I ask I was watching highlight from Chivas vs Houston and Chivas player gets to end line and crosses the ball. His body crosses over the endline by about 5-6 yards. The Houston defender (Hainault I think) drifts to slightly over the endline with one foot over. The wide open header was laughingly hit back to where the ball came from. The Chivas player comes back onto the field to get the ball in the field of play but was called offside. It didn't look like it went out of play. At the time the header was struck both players were over the endline which I would interpret as even. Am I wrong? http://www.mlssoccer.com/matchcenter/2012-03-11-chv-v-hou/highlights?videoID=178684
A player that leaves the field during normal course of play is to be considered to be on the line. If he deliberate leaves the field to he should be cautioned.
Your interpretation is correct. They are considered even. However, that's only one defender. The attacker is still beyond the goalkeeper and every other opponent. So he's offside. For a player off the field to ever be considered still in an onside position, there would have to be two defenders on the goal line (or also off the field).
Hainault, by being off the field, is considered to be on the goal line. Likewise for the Chivas attacker. A player is in an offside position if they are nearer to their opponents' goal line than the ball, the halfway line, and the second-to-last opponent. Hainault became the last defender, making another Houston player on the field the second-to-last defender. The Chivas attacker, therefore, was in an offside position at the time of the header. Had Houston had two players off the field at the time, only then would the Chivas attacker be "even" and therefore not in an offside position.
