The attribute-model sibling of the structural content-model check, folded into
the same schema-document walk. Enumerated attributes (form,
elementFormDefault, attributeFormDefault, use, processContents, mixed, abstract,
nillable) and the occurrence attributes (minOccurs a nonNegativeInteger,
maxOccurs a nonNegativeInteger or unbounded) are checked against their fixed
value space. An out-of-space value (use="foo", mixed="yes", minOccurs="-1") was
previously ignored and the schema accepted.
Each of these names has one meaning throughout XSD, so checking by local name is
safe. Only unprefixed attributes on XSD-namespace elements are checked, so a
foreign attribute is never flagged; values are trimmed before comparison.
XSTS: invalid schemas accepted 1313 to 1190 (-123); valid schemas rejected held
at 72 (no over-rejection); instance buckets unchanged.