But the laws specify what makes them
active [edited - sorry about that] - they specify what makes the player count, and are couched in wording about impacting ability of opponents to play the ball/obstructing vision etc.
There is no line of the offside law that is
clearly broken, just some where it comes down to an opinion of whether it was broken. Almost by definition, there is then no clear error, and VAR does nothing.
I think this is the relevant section:
A player in an offside position at the moment the ball is played or touched* by a team-mate is only penalised on becoming involved in active play by:
- interfering with play by playing or touching a ball passed or touched by a team-mate or [he didn't touch it]
- interfering with an opponent by:
- preventing an opponent from playing or being able to play the ball by clearly obstructing the opponent’s line of vision or [no City player was in range to play it, Ederson wasn't blocked]
- challenging an opponent for the ball or [didn't happen]
- clearly attempting to play a ball which is close when this action impacts on an opponent or [this is the point in most question for me, and is subjective]
- making an obvious action which clearly impacts on the ability of an opponent to play the ball [similar to the immediate point above]
or
- gaining an advantage by playing the ball or interfering with an opponent when it has:
- rebounded or been deflected off the goalpost, crossbar or an opponent [didn't happen]
- been deliberately saved by any opponent [didn't happen]
I think that particularly when Rashford cocked his left foot back as if to shoot, that was clearly impacted on Ederson as the ball would surely have gone to his right if Rashford shoots.
I think it was a terrible decision from a footballing opinion as it becomes impossible to defend against a player who is clearly in an offside position (see also the Salah goal vs Wolves recently), but I grew up when offside was simple, and now you get this.
I think if Rashford had stopped then Walker would have been more interested in challenging for the ball, but Rashford's continued running made him think that he must be impacting play.