2.1.1773 Part 4 Section 14.8.2.1, hdrShapeDefaults (Default Properties for VML Objects in Header and Footer)

a.   The standard states that any element from the VML namespace is a valid child element of the hdrShapeDefaults element.

Word only allows the shapedefaults and shapelayout elements from the VML namespace to be child elements of the hdrShapeDefaults element.