Better describe anchor restrictions in Draft 7 and Draft 6

content/draft6/core/id.markdown

@@ -1,7 +1,7 @@
 ---
 keyword: "$id"
 signature: "URI Reference"
-value: This keyword must be set to an absolute URI or a relative reference as defined by [RFC 3986](https://www.rfc-editor.org/info/rfc3986)
+value: "This keyword must be set to an absolute URI or a relative reference as defined by [RFC 3986](https://www.rfc-editor.org/info/rfc3986), where any fragment must match the regular expression `^[A-Za-z][-A-Za-z0-9.:_]*$`"
 summary: "This keyword declares an identifier for the schema resource."
 kind: [ "identifier" ]
 instance: [ "any" ]
@@ -54,6 +54,23 @@ identifiers.
 
 {{</common-pitfall>}}
 
+{{<common-pitfall>}}
+
+The specification restricts the kinds of fragments that may appear in
+[`$id`]({{< ref "draft6/core/id" >}}). Only _plain-name_ fragments (matching
+the regular expression `^[A-Za-z][-A-Za-z0-9.:_]*$`) are allowed, and they
+declare a location-independent identifier (an anchor) for the schema. JSON
+Pointer fragments such as `#/definitions/foo` are _not_ valid in
+[`$id`]({{< ref "draft6/core/id" >}}), even though they are valid in
+[`$ref`]({{< ref "draft6/core/ref" >}}). The behaviour of any other fragment
+form is undefined.
+
+This anchor-style usage of [`$id`]({{< ref "draft6/core/id" >}}) was later
+replaced by the dedicated [`$anchor`]({{< ref "2019-09/core/anchor" >}})
+keyword starting in JSON Schema 2019-09.
+
+{{</common-pitfall>}}
+
 {{<best-practice>}}
 
 It is strongly recommended for every schema file to explicitly declare an

content/draft7/core/id.markdown

@@ -1,7 +1,7 @@
 ---
 keyword: "$id"
 signature: "URI Reference"
-value: This keyword must be set to an absolute URI or a relative reference as defined by [RFC 3986](https://www.rfc-editor.org/info/rfc3986)
+value: "This keyword must be set to an absolute URI or a relative reference as defined by [RFC 3986](https://www.rfc-editor.org/info/rfc3986), where any fragment must match the regular expression `^[A-Za-z][-A-Za-z0-9.:_]*$`"
 summary: "This keyword declares an identifier for the schema resource."
 kind: [ "identifier" ]
 instance: [ "any" ]
@@ -54,6 +54,23 @@ identifiers.
 
 {{</common-pitfall>}}
 
+{{<common-pitfall>}}
+
+The specification restricts the kinds of fragments that may appear in
+[`$id`]({{< ref "draft7/core/id" >}}). Only _plain-name_ fragments (matching
+the regular expression `^[A-Za-z][-A-Za-z0-9.:_]*$`) are allowed, and they
+declare a location-independent identifier (an anchor) for the schema. JSON
+Pointer fragments such as `#/definitions/foo` are _not_ valid in
+[`$id`]({{< ref "draft7/core/id" >}}), even though they are valid in
+[`$ref`]({{< ref "draft7/core/ref" >}}). The behaviour of any other fragment
+form is undefined.
+
+This anchor-style usage of [`$id`]({{< ref "draft7/core/id" >}}) was later
+replaced by the dedicated [`$anchor`]({{< ref "2019-09/core/anchor" >}})
+keyword starting in JSON Schema 2019-09.
+
+{{</common-pitfall>}}
+
 {{<best-practice>}}
 
 It is strongly recommended for every schema file to explicitly declare an