For transform, transform-origin, and perspective-origin, Percentages says "refer to the size of the element's box".  This isn't clear: which box?  It should say "refer to the size of the element's border box", as in the merged Transforms spec.

(This is still not perfectly unambiguous, because it's left implicit that X values are relative to the width and Y values are relative to the height.  This is in contrast to the padding property, for instance, where all percentage values are relative to the width of the containing block.  But it's probably good enough.)