Editorial: Clarify consistency requirements for supported time zones

Adds notes in the appropriate places to prohibit cases such as time zones
becoming unsupported after they had previously been observed to be
supported, or becoming primary after they had previously been observed to
be primary.

Co-Authored-By: Richard Gibson <[email protected]>

Closes: #1921

Author: ptomato

spec/intl.html

@@ -132,7 +132,8 @@ <h1>AvailableNamedTimeZoneIdentifiers ( ): a List of Time Zone Identifier Record
 
         <emu-note>
           Time zone identifiers in the IANA Time Zone Database can change over time.
-          At a minimum, it is recommended that implementations limit changes to the result of AvailableNamedTimeZoneIdentifiers to the changes allowed by GetAvailableNamedTimeZoneIdentifier, for the lifetime of the surrounding agent.
+          At a minimum, implementations must limit changes to the result of AvailableNamedTimeZoneIdentifiers to the changes allowed by GetAvailableNamedTimeZoneIdentifier, for the lifetime of the surrounding agent.
+          That is, implementations must not remove support for or change the primary vs. non-primary status of an identifier that was already reported as available, and they must not add support for an identifier that was already reported as not available.
           Due to the complexity of supporting these recommendations, it is recommended that the result of AvailableNamedTimeZoneIdentifiers remains the same for the lifetime of the surrounding agent.
         </emu-note>
       </emu-clause>

spec/mainadditions.html

@@ -262,6 +262,13 @@ <h1>AvailableNamedTimeZoneIdentifiers ( ): a List of Time Zone Identifier Record
         1. Assert: _result_ contains a Time Zone Identifier Record _r_ such that _r_.[[Identifier]] is *"UTC"* and _r_.[[PrimaryIdentifier]] is *"UTC"*.
         1. Return _result_.
       </emu-alg>
+
+      <emu-note>
+        Time zone identifiers in the IANA Time Zone Database can change over time.
+        At a minimum, implementations must limit changes to the result of AvailableNamedTimeZoneIdentifiers to the changes allowed by GetAvailableNamedTimeZoneIdentifier, for the lifetime of the surrounding agent.
+        That is, implementations must not remove support for or change the primary vs. non-primary status of an identifier that was already reported as available, and they must not add support for an identifier that was already reported as not available.
+        Due to the complexity of supporting these recommendations, it is recommended that the result of AvailableNamedTimeZoneIdentifiers remains the same for the lifetime of the surrounding agent.
+      </emu-note>
     </emu-clause>
 
     <emu-clause id="sec-systemtimezoneidentifier" oldids="sec-defaulttimezone" type="implementation-defined abstract operation">

spec/timezone.html

@@ -544,9 +544,16 @@ <h1>
         1. Return ~empty~.
       </emu-alg>
       <emu-note>
-        For any _timeZoneIdentifier_, or any value that is an ASCII-case-insensitive match for it, the resulting Time Zone Identifier Record must contain the same field values for the lifetime of the surrounding agent.
-        Furthermore, time zone identifiers must not dynamically change from primary to non-primary during the lifetime of the surrounding agent, meaning that if _timeZoneIdentifier_ is an ASCII-case-insensitive match for the [[PrimaryIdentifier]] field of the result of a previous call to GetAvailableNamedTimeZoneIdentifier, then GetAvailableNamedTimeZoneIdentifier(_timeZoneIdentifier_) must return a record where [[Identifier]] is [[PrimaryIdentifier]].
-        Due to the complexity of supporting these requirements, it is recommended that the result of AvailableNamedTimeZoneIdentifiers (and therefore GetAvailableNamedTimeZoneIdentifier) remains the same for the lifetime of the surrounding agent.
+        <p>
+          For any _timeZoneIdentifier_, or any value that is an ASCII-case-insensitive match for it, the result of this operation must remain the same for the lifetime of the surrounding agent.
+          Specifically, if that result is a Time Zone Identifier Record, its fields must contain the same values.
+        </p>
+        <p>
+          Furthermore, time zone identifiers must not dynamically change from primary to non-primary or vice versa during the lifetime of the surrounding agent, meaning that if _timeZoneIdentifier_ is an ASCII-case-insensitive match for the [[PrimaryIdentifier]] field of the result of a previous call to GetAvailableNamedTimeZoneIdentifier, then GetAvailableNamedTimeZoneIdentifier(_timeZoneIdentifier_) must return a record where [[Identifier]] is [[PrimaryIdentifier]].
+        </p>
+        <p>
+          Due to the complexity of supporting these requirements, it is recommended that the result of AvailableNamedTimeZoneIdentifiers (and therefore GetAvailableNamedTimeZoneIdentifier) remains the same for the lifetime of the surrounding agent.
+        </p>
       </emu-note>
     </emu-clause>