MLS experimented with deviations from IFAB rules and standards in its early years, some of which had been used in the NASL and continue to be used in college soccer and many high-school conferences.
Among them was the use of a countdown clock — rather than a standard progressive clock — with time paused for dead ball situations at a referee's discretion. Halves ended when the clock reached 0:00, rather than at the whistle of the referee as was customary elsewhere.
Also implemented was the use of shootouts to resolve tie games. These best-of-five contests pitted a player 35 yards from goal with five seconds to put the ball past a goalkeeper; if needed the shootout headed into extra frames. A winning team received one standings point (as opposed to three for the regulation win).
While IFAB rules allow teams to substitute three players during games, MLS allowed a fourth, goalkeeper-only substitute. MLS discarded the rule after 2003 and adopted the IFAB standard, prompted in part by a match in which MetroStars coach Bob Bradley used a loophole to insert outfield player Eddie Gaven as a fourth substitute.
MLS eventually conceded that the rules changes, particularly the shootout, had alienated some traditional soccer fans while failing to draw new American sports fans as hoped. The shootout and countdown clock were eliminated after the 1999 season.
MLS continued to experiment with the settling of tie games in regular-season play. In 2000, a 10-minute golden-goal period replaced the shootout for tied games. It was abandoned after 2003. The golden-goal overtime remained through 2004 for playoff matches, where it had been used since the league's start.
In 2005 the league adopted a playoff extra-time structure that followed new IFAB standards for such situations: two full 15-minute periods, followed by penalty kicks if necessary. The away goals rule is not used in any playoff round.