[css-easing-1] Extrapolation of easing function

[kevers-google]

https://www.w3.org/TR/css-easing-1/#cubic-bezier-algo

For input progress < 0:
Point #2 implies that the tangent line though P0 and P2 should be used in the event that P1_x = 0 and P2_x > 0. This should be further constrained on P1_y = 0. It is only a valid tangent line if P1 and P0 are coincident.

If P0, P1 and P2 are all coincident. The tangent line should be through P0 and P3.

For input progress > 1:
Point #2 implies that the tangent line through P1 and P3 should be used in the event that P2_x = 1 and P1_x < 1. This should be further constrained on P2_y = 1. It is only a valid tangent line if P2 and P3 are coincident.

If P1, P2, and P3 are all coincident. The tangent line should be through P0 and P3.

[flackr]

This means that if P1 = (0, 1), we'll use the line (0, 0) through (0, 1) which has infinite slope as the tangent line. Should this result in an output progress of -Infinity or should we have another special case similar to point 3?