Right, but extents are really [minx, miny, maxx, maxy] - which in pixel space is [left, top, right, bottom].  Since we're dealing with pixel space here, if people think this is like an extent, they may be confused in a different way.

I was thinking the best idea would be to not pretend this is an extent.  Instead it is four padding values (in pixels).  We just need to be very clear about the order of those values.  And CSS sets a decent/guessable precedent in my opinion.