[css3-transitions] Back-tracking transition-timing-function

In the case that the points P1 and P2 were too over extended from (0,0) and
(1,1), respectively, the curve they generate could end up back-tracking to
some extent. In the event of some bad code that does this, how should the
user agent handle it? Should it simply skip from the lower branch of the
function to the higher branch? Should it readjust the values of P1 and P2 so
that the back-tracking is removed? Should it ignore the transition entirely?

In addition, should coordinates of P1 and P2 outside the [0-1] range be
allowed? There are some well formed cubic bezier curves with control points
outside the square that stay inside the square continuously.

~6 out of 5 statisticians say that the number of statistics that either make
no sense or use ridiculous timescales at all has dropped over 164% in the
last 5.62474396842 years.