Hi Bill,
I look to this section of the specification.
A resized ad must position itself such that the entire close event region appears onscreen. If
the container/SDK detects that a request to resize will result in the close event region being
offscreen, the container/SDK should return an error, and ignore the resize (e.g., leave the ad
in its current state).
If I understand your question correctly, the resized ad could set negative values, but if that results in the close region being offscreen then the request to mraid.resize() should trigger an error.
Thanks,
-Nathan