Gregsdennis/format registry

specs/jsonschema-validation.md

@@ -325,30 +325,40 @@ from previous iterations of this specification. Previously, `format` was an
 annotation-only keyword by default and implementations that supported assertion
 were required to offer some configuration that allowed users to explicitly
 enable assertion. Assertion is now a requirement in order to meet user
-expectations. See [json-schema-org/json-schema-spec
-#1520](https://github.com/json-schema-org/json-schema-spec/issues/1520) for
-more.
+expectations. See
+[json-schema-org/json-schema-spec #1520](https://github.com/json-schema-org/json-schema-spec/issues/1520)
+for more.
 
 In addition to the assertion behavior, this keyword also produces its value as
 an annotation.
 
 Implementations:
 
-- SHOULD provide validation for each format attribute defined in this
-  document;
-- MAY support format values not defined in this document, but such support MUST
-  be configurable and disabled by default;
+- SHOULD provide validation for each format attribute defined in this document;
+- are encouraged to provide validation for format attributes listed in the
+  {{format-registry}};
+- MAY support format values not defined in this document or listed in the
+  registry, but such support MUST be configurable and disabled by default;
 - SHOULD use a common parsing library or a well-known regular expression for
   each format;
-- SHOULD clearly document how and to what degree each format attribute is
-  validated.
+- SHOULD clearly document any limitations regarding format validation.
 
 The requirement for validation of format values in general is limited to
 syntactic checking; implementations SHOULD NOT attempt to send an email, connect
 to a URL, or otherwise check the existence of an entity identified by a format
 instance.
 
-#### Custom format values
+#### Format Registry {#format-registry}
+
+In addition to the formats defined by this document, JSON Schema also maintains
+a registry of formats defined by other specifications and organizations. As of
+the publication of this document, the format registry can be found at
+<https://github.com/json-schema-org/json-schema-spec/blob/main/specs/registries/format.json>.
+
+Implementations SHOULD support the formats listed in this registry as if they
+were defined by this document.
+
+#### Custom `format` Values
 
 Implementations MAY support custom format values. Save for agreement between
 parties, schema authors SHALL NOT expect a peer implementation to support such