Editorial: Stop requiring sensor permissions names to be a non-empty set.

Related to w3c/ambient-light#79, where we want to replace the
"ambient-light" powerful feature with an integration with the Media Capture
and Streams spec by only providing illuminance readouts if the same page has
an active local video source.

In this case, we rely on the Media Capture and Streams' permission model, as
having an active local video source implicitly means the "camera" permission
has already been granted.

Mention the Ambient Light Sensor case and change the requirement for the
snesor permissions name set.

Author: Raphael Kubo da Costa

index.bs

@@ -84,8 +84,12 @@ urlPrefix: https://w3c.github.io/geolocation-sensor/; spec: GEOLOCATION-SENSOR
 urlPrefix: https://w3c.github.io/proximity; spec: PROXIMITY
   type: dfn
     text: ProximitySensor; url: proximity-sensor-interface
+urlPrefix: https://w3c.github.io/mediacapture-main/; spec: MEDIACAPTURE-STREAMS
+  type: dfn
+    text: stopped; url: source-stopped
 </pre>
 <pre class=link-defaults>
+spec: mediacapture-streams; type:dfn; text:source
 spec: webidl; type:dfn; text:attribute
 spec: webidl; type:dfn; text:dictionary member
 spec: webidl; type:dfn; text:identifier
@@ -448,6 +452,11 @@ payment service from within an iframe.
 
 Access to [=sensor readings=] are controlled by the Permissions API [[!PERMISSIONS]].
 
+Note: in specific cases such as [[AMBIENT-LIGHT]], integration with the
+Permissions API is done indirectly -- illuminance readings are provided only
+when at least one local video [=source=] is not [=muted=] or [=stopped=]. By
+definition, that implies the <a permission>"camera"</a> permission has been granted.
+
 <h3 id="mitigation-strategies-case-by-case">Mitigation strategies applied on a case by case basis</h3>
 
 Each [=sensor type=] will need to be assessed individually,
@@ -778,10 +787,13 @@ A [=sensor type=] has a [=ordered set|set=] of <dfn export>associated sensors</d
 
 A [=sensor type=] may have a [=default sensor=].
 
-A [=sensor type=] has a [=set/is empty|nonempty=] [=ordered set|set=] of associated
-[=powerful feature/names=] referred to as <dfn export>sensor permission names</dfn>.
+A [=sensor type=] has a [=ordered set|set=] of associated [=powerful
+feature/names=] referred to as <dfn export>sensor permission names</dfn>.
 
 Note: multiple [=sensor types=] may share the same [=powerful feature/name=].
+In specific cases, [=extension specifications=] might choose to define an
+[=set/empty=] set of [=sensor permission names=] if their permission model is
+integrated with that of another specification.
 
 A [=sensor type=] has a [=permission revocation algorithm=].
 
@@ -2105,6 +2117,11 @@ for instance, "gyroscope" or "accelerometer". [=sensor fusion|Fusion sensors=] m
 [=request permission to use|request permission to access=] each of the sensors that are
 used as a source of fusion.
 
+Note: see the note in [[#permissions]]. In specific cases such as
+[[AMBIENT-LIGHT]], an [=extension specification=] may choose to rely on
+[=powerful feature/names=] defined in another specification rather than
+specifying their own [=sensor permission names=].
+
 Even though it might be difficult to reconstruct [=low-level=] [=sensor readings=] from
 fused data, some of the original information might be inferred. For example, it is easy to
 deduce user's orientation in space if absolute or geomagnetic orientation sensors are used,