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