The step property is very strange in elm Slider , it is clamped in the range 0.0-1.0
seems it is threated as a percentage and not like a normal values like it should be.
For example is not possible to setup a slider as:
min=0 max=100 step=5
in fact in the elm test there is a nasty function _step_size_calculate() that try to avoid the issue.
Also the step property is not taken in account when you drag the slider with the mouse, it only work with keyboard (as the doc say).
IMO this property should be fixed to behave "normally" and work also when dragging.