Recommend registries, and give some guidance on how to define them.

index.bs

@@ -3567,6 +3567,80 @@ and makes it clear when the state described by the flags is reset.
 
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+<h3 id="registries">Use registries to allow constrained extensions outside the original standards track</h3>
+
+When you expect a feature to need to be extended over time,
+and it would be inappropriate to
+manage that change through the same standards process that defined the original feature,
+then a registry is usually the right design.
+
+[[spec-variability#variability|"Variability complicates interoperability."]]
+When a feature needs optional components,
+it is easiest to manage the complication if
+a single group is in charge of the whole design.
+This implies that most optional features should be defined
+in the same specification that defined the original feature.
+
+Sometimes new extensions are expected to be needed in the future.
+In this case, the right plan is often to
+have the original standards body continue or restart
+a working group to produce an update to the feature's full specification.
+
+However, if extensions need to be defined through
+a different process than updating the full specification,
+a registry is usually the right way to manage the known extensions.
+Default to defining either
+an IANA registry governed by [[RFC8126]] or a [[w3c-process#registries|W3C Registry]],
+but another similar mechanism is also acceptable.
+
+Registries are helpful because they
+
+1. Reduce name collisions.
+1. Help readers find the name for a desired purpose.
+1. Direct readers to a specification for each name
+    (if the registry requires this).
+
+Items in a registry generally need to implement a defined interface
+in order to plug into the specification they're extending.
+The definition of the registry must be clear about this interface,
+and it should require new entries to include a specification that follows it.
+This roughly corresponds to
+the IETF's [[RFC8126#section-4.6|Specification Required]] registration policy.
+It's difficult to be confident in an interface definition
+before that interface has several implementations,
+so any new registry should start with at least 2-3 entries defined.
+
+It is tempting to additionally require that registry entries be
+"good" in some way beyond simply being specified.
+Use a permissive registration policy instead of doing this
+unless the feature includes
+some way to enforce that no implementation can use an unregistered name.
+The IETF has found that
+when it's too hard to add entries to a registry,
+implementers will often simply use names without registering them.
+
+When a feature's main specification defines some initial registry entries,
+each one can be either required or optional for implementations to recognize.
+If implementations need to pick an understood registry entry to send to their recipients,
+then the registry should include at least 1 required entry.
+If implementations can be
+divided into distinct [[spec-variability#subdivision-profile|profiles]]
+that tend not to need to communicate with other profiles,
+it can be sufficient for each profile to require a registry entry
+even if the main specification leaves them all optional.
+
+It can be tempting to use URLs instead of registered strings to identify extensions.
+This has the benefit of making it very easy to extend the feature,
+by just picking a URL from a domain that the extender controls.
+However, there's no way to ensure that
+these URLs point to a specification for the extension they name,
+or even that the domain stays under the control of the entity that defined the extension.
+URLs are appropriate for a few kinds of completely-permissionless extension,
+but most of the time a WG-managed permissive registry table will work better.
+
+See [[qaframe-spec#extensions]] for
+more guidance on how to design extensibility.
+
 <h3 id="implementability">Resolving tension between interoperability and implementability</h3>
 
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