From the spec:
Value: <length> | <percentage>
Applies to: floats
Computed value: the absolute length
Animatable: as length, percentage, or calc.
The computed value is returned as the original percentage.
Created attachment 230381 [details]
Test case for bug
It turns out that this one is not valid.
W3C thread: http://lists.w3.org/Archives/Public/www-style/2014Apr/0475.html
Altered spec: http://dev.w3.org/csswg/css-shapes/#shape-margin-property