[css-rhythm-1] Define interaction of step size and floats #1901

Since vertical position influences available width in the presence of floats, this can create a weird feedback loop in determining the correct addition of space for step sizing. Spec should define how to break this loop.

Constraints that should be honored: The total addition of space should be between zero and the step size. Since this is probably more important than the exact distribution of space (top, bottom, both equally), the alignment should be relaxed in order to limit the addition of space to < one step size.