diff --git a/README.md b/README.md
index 7fc6d635..a8d9f7da 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,9 @@
 # Net::IMAP
 
 Net::IMAP implements Internet Message Access Protocol (IMAP) client
-functionality.  The protocol is described in [RFC3501](https://www.rfc-editor.org/rfc/rfc3501).
+functionality.  The protocol is described in
+[RFC3501](https://www.rfc-editor.org/rfc/rfc3501),
+[RFC9051](https://www.rfc-editor.org/rfc/rfc9051) and various extensions.
 
 ## Installation
 
diff --git a/docs/styles.css b/docs/styles.css
index e945aea0..1d1e27f1 100644
--- a/docs/styles.css
+++ b/docs/styles.css
@@ -10,18 +10,28 @@ main .method-detail {
   justify-content: space-between;
 }
 
-main .method-header, main .method-controls {
+main .method-header,
+main .method-controls,
+.attribute-method-heading {
   padding: 0.5em;
   /* border: 1px solid var(--highlight-color); */
   background: var(--table-header-background-color);
   line-height: 1.6;
 }
 
+.attribute-method-heading .attribute-access-type {
+  float: right;
+}
+
 main .method-header {
   border-right: none;
   border-radius: 4px 0 0 4px;
 }
 
+main .method-heading :any-link {
+  text-decoration: none;
+}
+
 main .method-controls {
   border-left: none;
   border-radius: 0 4px 4px 0;
diff --git a/lib/net/imap/config.rb b/lib/net/imap/config.rb
index 7532737c..edeea30e 100644
--- a/lib/net/imap/config.rb
+++ b/lib/net/imap/config.rb
@@ -75,7 +75,7 @@ class IMAP
     #
     #   client = Net::IMAP.new(hostname, config: :future)
     #   client.config.sasl_ir                  # => true
-    #   client.config.responses_without_block  # => :raise
+    #   client.config.responses_without_block  # => :frozen_dup
     #
     # The versioned default configs inherit certain specific config options from
     # Config.global, for example #debug:
@@ -109,9 +109,11 @@ class IMAP
     # [+:future+]
     #   The _planned_ eventual config for some future +x.y+ version.
     #
-    # For example, to raise exceptions for all current deprecations:
+    # For example, to disable all currently deprecated behavior:
     #   client = Net::IMAP.new(hostname, config: :future)
-    #   client.responses  # raises an ArgumentError
+    #   client.config.response_without_args     # => :frozen_dup
+    #   client.responses.frozen?                # => true
+    #   client.responses.values.all?(&:frozen?) # => true
     #
     # == Thread Safety
     #
diff --git a/lib/net/imap/fetch_data.rb b/lib/net/imap/fetch_data.rb
index 3d8554b5..271063ab 100644
--- a/lib/net/imap/fetch_data.rb
+++ b/lib/net/imap/fetch_data.rb
@@ -103,6 +103,15 @@ class IMAP < Protocol
     #   as <tt>BODY[#{specifier}]</tt> or <tt>BINARY[#{section}]</tt>.
     #
     class FetchStruct < Struct
+      ##
+      # method: attr
+      # :call-seq: attr -> hash
+      #
+      # Each key specifies a message attribute, and the value is the
+      # corresponding data item.  Standard data items have corresponding
+      # accessor methods.  The definitions of each attribute type is documented
+      # on its accessor.
+
       # :call-seq: attr_upcase -> hash
       #
       # A transformation of #attr, with all the keys converted to upper case.
diff --git a/lib/net/imap/response_data.rb b/lib/net/imap/response_data.rb
index 108ff29b..16ff306c 100644
--- a/lib/net/imap/response_data.rb
+++ b/lib/net/imap/response_data.rb
@@ -160,13 +160,20 @@ class TaggedResponse < Struct.new(:tag, :name, :data, :raw_data)
       # The raw response data.
     end
 
-    # Net::IMAP::ResponseText represents texts of responses.
+    # ResponseText represents texts of responses.
     #
     # The text may be prefixed by a ResponseCode.
     #
-    # ResponseText is returned from TaggedResponse#data, or from
-    # UntaggedResponse#data when the response type is a "condition" ("OK", "NO",
-    # "BAD", "PREAUTH", or "BYE").
+    # ResponseText is returned from TaggedResponse#data or
+    # UntaggedResponse#data for
+    # {"status responses"}[https://www.rfc-editor.org/rfc/rfc3501#section-7.1]:
+    # * every TaggedResponse, name[rdoc-ref:TaggedResponse#name] is always
+    #   "+OK+", "+NO+", or "+BAD+".
+    # * any UntaggedResponse when name[rdoc-ref:UntaggedResponse#name] is
+    #   "+OK+", "+NO+", "+BAD+", "+PREAUTH+", or "+BYE+".
+    #
+    # Note that these "status responses" are confusingly _not_ the same as the
+    # +STATUS+ UntaggedResponse (see IMAP#status and StatusData).
     class ResponseText < Struct.new(:code, :text)
       # Used to avoid an allocation when ResponseText is empty
       EMPTY = new(nil, "").freeze
@@ -184,32 +191,35 @@ class ResponseText < Struct.new(:code, :text)
       # Returns the response text, not including any response code
     end
 
-    # Net::IMAP::ResponseCode represents response codes.  Response codes can be
-    # retrieved from ResponseText#code and can be included in any "condition"
-    # response: any TaggedResponse and UntaggedResponse when the response type
-    # is a "condition" ("OK", "NO", "BAD", "PREAUTH", or "BYE").
+    # ResponseCode represents an \IMAP response code, which can be retrieved
+    # from ResponseText#code for
+    # {"status responses"}[https://www.rfc-editor.org/rfc/rfc3501#section-7.1]:
+    # * every TaggedResponse, name[rdoc-ref:TaggedResponse#name] is always
+    #   "+OK+", "+NO+", or "+BAD+".
+    # * any UntaggedResponse when name[rdoc-ref:UntaggedResponse#name] is
+    #   "+OK+", "+NO+", "+BAD+", "+PREAUTH+", or "+BYE+".
+    #
+    # Note that these "status responses" are confusingly _not_ the same as the
+    # +STATUS+ UntaggedResponse (see IMAP#status and StatusData).
     #
     # Some response codes come with additional data which will be parsed by
     # Net::IMAP.  Others return +nil+ for #data, but are used as a
     # machine-readable annotation for the human-readable ResponseText#text in
-    # the same response.  When Net::IMAP does not know how to parse response
-    # code text, #data returns the unparsed string.
+    # the same response.
     #
     # Untagged response code #data is pushed directly onto Net::IMAP#responses,
     # keyed by #name, unless it is removed by the command that generated it.
     # Use Net::IMAP#add_response_handler to view tagged response codes for
     # command methods that do not return their TaggedResponse.
     #
+    # == Standard response codes
+    #
     # \IMAP extensions may define new codes and the data that comes with them.
     # The IANA {IMAP Response
     # Codes}[https://www.iana.org/assignments/imap-response-codes/imap-response-codes.xhtml]
     # registry has links to specifications for all standard response codes.
-    # Response codes are backwards compatible:  Servers are allowed to send new
-    # response codes even if the client has not enabled the extension that
-    # defines them.  When unknown response code data is encountered, #data
-    # will return an unparsed string.
     #
-    # ==== +IMAP4rev1+ Response Codes
+    # === +IMAP4rev1+ response codes
     # See [IMAP4rev1[https://www.rfc-editor.org/rfc/rfc3501]] {§7.1, "Server
     # Responses - Status
     # Responses"}[https://www.rfc-editor.org/rfc/rfc3501#section-7.1] for full
@@ -240,24 +250,24 @@ class ResponseText < Struct.new(:code, :text)
     #   the <tt>\Seen</tt> flag set.
     #   <em>DEPRECATED by IMAP4rev2.</em>
     #
-    # ==== +BINARY+ extension
+    # === +BINARY+ extension
     # See {[RFC3516]}[https://www.rfc-editor.org/rfc/rfc3516].
     # * +UNKNOWN-CTE+, with a tagged +NO+ response, when the server does not
     #   known how to decode a CTE (content-transfer-encoding).  #data is +nil+.
     #   See IMAP#fetch.
     #
-    # ==== +UIDPLUS+ extension
+    # === +UIDPLUS+ extension
     # See {[RFC4315 §3]}[https://www.rfc-editor.org/rfc/rfc4315#section-3].
     # * +APPENDUID+, #data is UIDPlusData.  See IMAP#append.
     # * +COPYUID+, #data is UIDPlusData.  See IMAP#copy.
     # * +UIDNOTSTICKY+, #data is +nil+.  See IMAP#select.
     #
-    # ==== +SEARCHRES+ extension
+    # === +SEARCHRES+ extension
     # See {[RFC5182]}[https://www.rfc-editor.org/rfc/rfc5182].
     # * +NOTSAVED+, with a tagged +NO+ response, when the search result variable
     #   is not saved.  #data is +nil+.
     #
-    # ==== +RFC5530+ Response Codes
+    # === +RFC5530+ response codes
     # See {[RFC5530]}[https://www.rfc-editor.org/rfc/rfc5530], "IMAP Response
     # Codes" for the definition of the following response codes, which are all
     # machine-readable annotations for the human-readable ResponseText#text, and
@@ -280,22 +290,22 @@ class ResponseText < Struct.new(:code, :text)
     # * +ALREADYEXISTS+
     # * +NONEXISTENT+
     #
-    # ==== +QRESYNC+ extension
+    # === +QRESYNC+ extension
     # See {[RFC7162]}[https://www.rfc-editor.org/rfc/rfc7162.html].
     # * +CLOSED+, returned when the currently selected mailbox is closed
     #   implicitly by selecting or examining another mailbox.  #data is +nil+.
     #
-    # ==== +IMAP4rev2+ Response Codes
+    # === +IMAP4rev2+ response codes
     # See {[RFC9051]}[https://www.rfc-editor.org/rfc/rfc9051] {§7.1, "Server
     # Responses - Status
     # Responses"}[https://www.rfc-editor.org/rfc/rfc9051#section-7.1] for full
     # descriptions of IMAP4rev2 response codes.  IMAP4rev2 includes all of the
-    # response codes listed above (except "UNSEEN") and adds the following:
+    # response codes listed above (except "+UNSEEN+") and adds the following:
     # * +HASCHILDREN+, with a tagged +NO+ response, when a mailbox delete failed
     #   because the server doesn't allow deletion of mailboxes with children.
     #   #data is +nil+.
     #
-    # ==== +CONDSTORE+ extension
+    # === +CONDSTORE+ extension
     # See {[RFC7162]}[https://www.rfc-editor.org/rfc/rfc7162.html].
     # * +NOMODSEQ+, when selecting a mailbox that does not support
     #   mod-sequences.  #data is +nil+.  See IMAP#select.
@@ -305,10 +315,17 @@ class ResponseText < Struct.new(:code, :text)
     #   since the +UNCHANGEDSINCE+ mod-sequence given to +STORE+ or <tt>UID
     #   STORE</tt>.
     #
-    # ==== +OBJECTID+ extension
+    # === +OBJECTID+ extension
     # See {[RFC8474]}[https://www.rfc-editor.org/rfc/rfc8474.html].
     # * +MAILBOXID+, #data is a string
     #
+    # == Extension compatibility
+    #
+    # Response codes are backwards compatible:  Servers are allowed to send new
+    # response codes even if the client has not enabled the extension that
+    # defines them.  When Net::IMAP does not know how to parse response
+    # code text, #data returns the unparsed string.
+    #
     class ResponseCode < Struct.new(:name, :data)
       ##
       # method: name
@@ -327,14 +344,9 @@ class ResponseCode < Struct.new(:name, :data)
       # code data can take.
     end
 
-    # Net::IMAP::UIDPlusData represents the ResponseCode#data that accompanies
-    # the +APPENDUID+ and +COPYUID+ response codes.
-    #
-    # See [[UIDPLUS[https://www.rfc-editor.org/rfc/rfc4315.html]].
-    #
-    # ==== Capability requirement
+    # UIDPlusData represents the ResponseCode#data that accompanies the
+    # +APPENDUID+ and +COPYUID+ {response codes}[rdoc-ref:ResponseCode].
     #
-    # The +UIDPLUS+ capability[rdoc-ref:Net::IMAP#capability] must be supported.
     # A server that supports +UIDPLUS+ should send a UIDPlusData object inside
     # every TaggedResponse returned by the append[rdoc-ref:Net::IMAP#append],
     # copy[rdoc-ref:Net::IMAP#copy], move[rdoc-ref:Net::IMAP#move], {uid
@@ -342,9 +354,9 @@ class ResponseCode < Struct.new(:name, :data)
     # move}[rdoc-ref:Net::IMAP#uid_move] commands---unless the destination
     # mailbox reports +UIDNOTSTICKY+.
     #
-    #--
-    # TODO: support MULTIAPPEND
-    #++
+    # == Required capability
+    # Requires either +UIDPLUS+ [RFC4315[https://www.rfc-editor.org/rfc/rfc4315]]
+    # or +IMAP4rev2+ capability.
     #
     class UIDPlusData < Struct.new(:uidvalidity, :source_uids, :assigned_uids)
       ##
@@ -381,10 +393,8 @@ def uid_mapping
       end
     end
 
-    # Net::IMAP::MailboxList represents contents of the LIST response,
-    # representing a single mailbox path.
-    #
-    # Net::IMAP#list returns an array of MailboxList objects.
+    # MailboxList represents the data of an untagged +LIST+ response, for a
+    # _single_ mailbox path.  IMAP#list returns an array of MailboxList objects.
     #
     class MailboxList < Struct.new(:attr, :delim, :name)
       ##
@@ -394,11 +404,11 @@ class MailboxList < Struct.new(:attr, :delim, :name)
       # Returns the name attributes. Each name attribute is a symbol capitalized
       # by String#capitalize, such as :Noselect (not :NoSelect).  For the
       # semantics of each attribute, see:
-      # * rdoc-ref:Net::IMAP@Basic+Mailbox+Attributes
-      # * rdoc-ref:Net::IMAP@Mailbox+role+Attributes
-      # * Net::IMAP@SPECIAL-USE
+      # * Net::IMAP@Basic+Mailbox+Attributes
+      # * Net::IMAP@Mailbox+role+Attributes
       # * The IANA {IMAP Mailbox Name Attributes
       #   registry}[https://www.iana.org/assignments/imap-mailbox-name-attributes/imap-mailbox-name-attributes.xhtml]
+      #   has links to specifications for all standard mailbox attributes.
 
       ##
       # method: delim
@@ -413,16 +423,16 @@ class MailboxList < Struct.new(:attr, :delim, :name)
       # Returns the mailbox name.
     end
 
-    # Net::IMAP::MailboxQuota represents contents of GETQUOTA response.
-    # This object can also be a response to GETQUOTAROOT.  In the syntax
-    # specification below, the delimiter used with the "#" construct is a
-    # single space (SPACE).
+    # MailboxQuota represents the data of an untagged +QUOTA+ response.
     #
-    # Net:IMAP#getquota returns an array of MailboxQuota objects.
+    # IMAP#getquota returns an array of MailboxQuota objects.
     #
     # Net::IMAP#getquotaroot returns an array containing both MailboxQuotaRoot
     # and MailboxQuota objects.
     #
+    # == Required capability
+    # Requires +QUOTA+ [RFC2087[https://www.rfc-editor.org/rfc/rfc2087]]
+    # capability.
     class MailboxQuota < Struct.new(:mailbox, :usage, :quota)
       ##
       # method: mailbox
@@ -444,12 +454,14 @@ class MailboxQuota < Struct.new(:mailbox, :usage, :quota)
       #
     end
 
-    # Net::IMAP::MailboxQuotaRoot represents part of the GETQUOTAROOT
-    # response. (GETQUOTAROOT can also return Net::IMAP::MailboxQuota.)
+    # MailboxQuotaRoot represents the data of an untagged +QUOTAROOT+ response.
     #
-    # Net::IMAP#getquotaroot returns an array containing both MailboxQuotaRoot
-    # and MailboxQuota objects.
+    # IMAP#getquotaroot returns an array containing both MailboxQuotaRoot and
+    # MailboxQuota objects.
     #
+    # == Required capability
+    # Requires +QUOTA+ [RFC2087[https://www.rfc-editor.org/rfc/rfc2087]]
+    # capability.
     class MailboxQuotaRoot < Struct.new(:mailbox, :quotaroots)
       ##
       # method: mailbox
@@ -464,12 +476,13 @@ class MailboxQuotaRoot < Struct.new(:mailbox, :quotaroots)
       # Zero or more quotaroots that affect the quota on the specified mailbox.
     end
 
-    # Net::IMAP::MailboxACLItem represents the response from GETACL.
+    # MailboxACLItem represents the data of an untagged +ACL+ response.
     #
-    # Net::IMAP#getacl returns an array of MailboxACLItem objects.
+    # IMAP#getacl returns an array of MailboxACLItem objects.
     #
-    # ==== Required capability
-    # +ACL+ - described in [ACL[https://www.rfc-editor.org/rfc/rfc4314]]
+    # == Required capability
+    # Requires +ACL+ [RFC4314[https://www.rfc-editor.org/rfc/rfc4314]]
+    # capability.
     class MailboxACLItem < Struct.new(:user, :rights, :mailbox)
       ##
       # method: mailbox
@@ -491,11 +504,12 @@ class MailboxACLItem < Struct.new(:user, :rights, :mailbox)
       # The access rights the indicated #user has to the #mailbox.
     end
 
-    # Net::IMAP::Namespace represents a single namespace contained inside a
-    # NAMESPACE response.
-    #
-    # Returned by Net::IMAP#namespace, contained inside a Namespaces object.
+    # Namespace represents a _single_ namespace, contained inside a Namespaces
+    # object.
     #
+    # == Required capability
+    # Requires either +NAMESPACE+ [RFC2342[https://www.rfc-editor.org/rfc/rfc2342]]
+    # or +IMAP4rev2+ capability.
     class Namespace < Struct.new(:prefix, :delim, :extensions)
       ##
       # method: prefix
@@ -517,11 +531,14 @@ class Namespace < Struct.new(:prefix, :delim, :extensions)
       # Extension parameter semantics would be defined by the extension.
     end
 
-    # Net::IMAP::Namespaces represents a +NAMESPACE+ server response, which
-    # contains lists of #personal, #shared, and #other namespaces.
+    # Namespaces represents the data of an untagged +NAMESPACE+ response,
+    # returned by IMAP#namespace.
     #
-    # Net::IMAP#namespace returns a Namespaces object.
+    # Contains lists of #personal, #shared, and #other namespaces.
     #
+    # == Required capability
+    # Requires either +NAMESPACE+ [RFC2342[https://www.rfc-editor.org/rfc/rfc2342]]
+    # or +IMAP4rev2+ capability.
     class Namespaces < Struct.new(:personal, :other, :shared)
       ##
       # method: personal
@@ -542,9 +559,9 @@ class Namespaces < Struct.new(:personal, :other, :shared)
       # Returns an array of Shared Namespace objects.
     end
 
-    # Net::IMAP::StatusData represents the contents of the STATUS response.
+    # StatusData represents the contents of an untagged +STATUS+ response.
     #
-    # Net::IMAP#status returns the contents of #attr.
+    # IMAP#status returns the contents of #attr.
     class StatusData < Struct.new(:mailbox, :attr)
       ##
       # method: mailbox
@@ -695,7 +712,7 @@ class Envelope < Struct.new(:date, :subject, :from, :sender, :reply_to,
     # parsed into its component parts by the server.  Address objects are
     # returned within Envelope fields.
     #
-    # === Group syntax
+    # == Group syntax
     #
     # When the #host field is +nil+, this is a special form of address structure
     # that indicates the [RFC5322[https://www.rfc-editor.org/rfc/rfc5322]] group
@@ -802,7 +819,7 @@ def all_seqnos(node = self)
     # for full description of all +BODYSTRUCTURE+ fields, and also
     # Net::IMAP@Message+envelope+and+body+structure for other relevant RFCs.
     #
-    # === Classes that include BodyStructure
+    # == Classes that include BodyStructure
     # BodyTypeBasic:: Represents any message parts that are not handled by
     #                 BodyTypeText, BodyTypeMessage, or BodyTypeMultipart.
     # BodyTypeText:: Used by <tt>text/*</tt> parts.  Contains all of the