Stair treads must be resistant. this is especially important for outside stairs exposed to the elements or stairways exposed to wet conditions.

There must be a stairway or ladder at points of access where there is an ... Must have uniform riser height and tread depth, with less than a 1/4-inch variation. ... Ladders must be kept in a safe condition ... If using ladders where the employee or the ladder could contact exposed energized electrical equipment

The answer for that question is  {stair treads must be slip resistant