[web-animations] compound longhand interpolation

Hi list,

What should the result of interpolating (e.g. for clip):
rect(0px, 0px, 0px, auto)
to
rect(100px, 100px, 100px, 50px)
be?

What about (e.g. for mask-box-image-slice):
50% fill
to
30%

Should individual longhand properties with multiple 'channels' interpolate
each channel separately (i.e. 0px can smoothly interpolate to 100px even
while auto steps to 50px), or should a single non-interpolating channel
block interpolation of all channels (i.e. fill -> not fill is a step, so
50% to 30% should step too)?

A relevant precedent may be that repeatable lists will not interpolate if
any pair is not interpolable.

Cheers,
    -Shane