An xs:all group in a complexContent extension of a base that already has element
content was accepted. The effective content model of such an extension is
sequence(base content, extension content), which nests the all and violates the
rule that an all group must be the whole content model. The base type is read
from the compiled model: rejected only when the base resolves, in the schema's
own target namespace, to a complex type with element-only or mixed content. An
explicitly-empty base (an empty model group with mixed=false) has effectively
empty content and may still be extended by an all (the all becomes the whole
content); a simple-content, empty, foreign, or unresolved base is left alone.
Checked for a self-contained schema.
XSTS: invalid schemas accepted 411 to 403, no other bucket raised.