An element with a default or fixed value constraint must have a content type
that admits character data: a simple type, or mixed content (cos-valid-default
/ e-props-correct). An element whose type is, or whose inline complex type has,
element-only or empty content may not carry a value constraint.
elementValueConstraintContentErrors flags only a content type that is certainly
element-only or empty; a simple type, mixed content, xs:anyType (mixed), an
untyped element (anyType), a complexContent-derived type (mixedness may be
inherited), and a foreign or unresolved type are all left alone, so it cannot
over-reject.
XSTS: invalid-schemas-accepted 99 to 96 (elemG003/elemG004/elemD004),
valid-schemas-rejected held at 0, no other bucket moved. Differential:
PureXML-stricter unchanged at 87 over 9618 schemas.