require $schema in schemas

jsonschema-core.md

@@ -884,25 +884,37 @@ meta-schema features emphasizes flexibility over simplicity.
 
 The `$schema` keyword is both used as a JSON Schema dialect identifier and as
 the identifier of a resource which is itself a JSON Schema, which describes the
-set of valid schemas written for this particular dialect.
+set of valid schemas written for this particular dialect. The identified dialect
+applies to the resource in which it is declared as well as any embedded schema
+resources, unless such a resource itself declares a different dialect by
+including the `$schema` keyword with a different value.
 
-The value of this keyword MUST be a [IRI](#rfc3987) (containing a scheme) and
+The value of this keyword MUST be an [IRI](#rfc3987) (containing a scheme) and
 this IRI MUST be normalized. The current schema MUST be valid against the
 meta-schema identified by this IRI.
 
 If this IRI identifies a retrievable resource, that resource SHOULD be of media
 type `application/schema+json`.
 
 The `$schema` keyword SHOULD be used in the document root schema object, and MAY
-be used in the root schema objects of embedded schema resources. It MUST NOT
-appear in non-resource root schema objects. If absent from the document root
-schema, the resulting behavior is implementation-defined, but MUST fall within
-the following options:
-
-- Refuse to process the schema, as with unsupported required vocabularies
-- Assume a specific, documented meta-schema
-- Document the process by which it examines the schema and determines which of a
-  specific set of meta-schemas to assume
+be used in the root schema objects of embedded schema resources. When the
+keyword appears in non-resource root schema object, the behavior is undefined.
+
+If present in the resource root schema, an implementation MUST process the
+schema in accordance with the associated dialect.
+
+If absent from the resource root schema, and no parent dialect is defined, the
+schema MUST be processed with the following priorities:
+
+1. If the implementation accepts media type parameter inputs, and an
+   `application/schema+json` media type with a `schema` parameter is available,
+   then the schema MUST be processed in accordance with the dialect indicated by
+   the `schema` parameter.
+2. An implementation MAY offer a mechanism for the user to explicitly set the
+   dialect.
+
+If the dialect is not specified through one of these methods, the implementation
+MUST refuse to process the schema, as with unsupported required vocabularies.
 
 Values for this property are defined elsewhere in this and other documents, and
 by other parties.