Add native origins for reference spaces

index.bs

@@ -1324,15 +1324,17 @@ Each {{XRReferenceSpace}} has a <dfn for="XRReferenceSpace">type</dfn>, which is
 
 An {{XRReferenceSpace}} is most frequently obtained by calling {{XRSession/requestReferenceSpace()}}, which creates an instance of an {{XRReferenceSpace}} (or an interface extending it) if the {{XRReferenceSpaceType}} enum value passed into the call [=reference space is supported|is supported=]. The type indicates the tracking behavior that the reference space will exhibit:
 
-  - Passing a type of <dfn enum-value for="XRReferenceSpaceType">viewer</dfn> creates an {{XRReferenceSpace}} instance. It represents a tracking space with a [=native origin=] which tracks the position and orientation of the [=viewer=]. Every {{XRSession}} MUST support {{XRReferenceSpaceType/"viewer"}} {{XRReferenceSpace}}s.
+  - Passing a type of <dfn enum-value for="XRReferenceSpaceType">viewer</dfn> creates an {{XRReferenceSpace}} instance. It represents a tracking space with a [=native origin=] which tracks the position and orientation of the [=viewer=], with `-Z` pointing forwards, along the viewer's gaze, `+Y` pointing upwards in the viewer's perception, and `+X` pointing to their right. Every {{XRSession}} MUST support {{XRReferenceSpaceType/"viewer"}} {{XRReferenceSpace}}s.
 
-  - Passing a type of <dfn enum-value for="XRReferenceSpaceType">local</dfn> creates an {{XRReferenceSpace}} instance. It represents a tracking space with a [=native origin=] near the viewer at the time of creation. The exact position and orientation will be initialized based on the conventions of the underlying platform. When using this reference space the user is not expected to move beyond their initial position much, if at all, and tracking is optimized for that purpose. For devices with [=6DoF=] tracking, {{local}} reference spaces should emphasize keeping the origin stable relative to the user's environment.
+  - Passing a type of <dfn enum-value for="XRReferenceSpaceType">local</dfn> creates an {{XRReferenceSpace}} instance. It represents a tracking space with a [=native origin=] near the viewer at the time of creation, with `+Y` pointing upwards. The exact position and orientation will be initialized based on the conventions of the underlying platform. When using this reference space the user is not expected to move beyond their initial position much, if at all, and tracking is optimized for that purpose. For devices with [=6DoF=] tracking, {{local}} reference spaces should emphasize keeping the origin stable relative to the user's environment.
 
-  - Passing a type of <dfn enum-value for="XRReferenceSpaceType">local-floor</dfn> creates an {{XRReferenceSpace}} instance. It represents a tracking space with a [=native origin=] at the floor in a safe position for the user to stand. The `Y` axis equals `0` at floor level, with the `X` and `Z` position and orientation initialized based on the conventions of the underlying platform. If the floor level isn't known it MUST be estimated, with some <dfn>estimated floor level</dfn>. If the [=estimated floor level=] is determined with a non-default value, it MUST be [=rounding|rounded=] sufficiently to prevent fingerprinting. When using this reference space the user is not expected to move beyond their initial position much, if at all, and tracking is optimized for that purpose. For devices with [=6DoF=] tracking, {{local-floor}} reference spaces should emphasize keeping the origin stable relative to the user's environment.
+   Note: On most platforms, the [=native origin=] of {{XRReferenceSpaceType/"local"}} will be identical to the position and orientation of {{XRReferenceSpaceType/"viewer"}} at time of session creation.
+
+  - Passing a type of <dfn enum-value for="XRReferenceSpaceType">local-floor</dfn> creates an {{XRReferenceSpace}} instance. It represents a tracking space with a [=native origin=] at the floor in a safe position for the user to stand. The `Y` axis equals `0` at floor level (`+Y` pointing upwards), with the `X` and `Z` position and orientation initialized based on the conventions of the underlying platform. <span class=note>On most platforms, this will be with `+X` pointing towards the right of the viewer at time of session creation, and `-Z` pointing forwards.</span> If the floor level isn't known it MUST be estimated, with some <dfn>estimated floor level</dfn>. If the [=estimated floor level=] is determined with a non-default value, it MUST be [=rounding|rounded=] sufficiently to prevent fingerprinting. When using this reference space the user is not expected to move beyond their initial position much, if at all, and tracking is optimized for that purpose. For devices with [=6DoF=] tracking, {{local-floor}} reference spaces should emphasize keeping the origin stable relative to the user's environment.
 
     Note: If the floor level of a {{XRReferenceSpaceType/"local-floor"}} reference space is adjusted to prevent fingerprinting, [=rounding|rounded=] to the nearest 1cm is suggested.
 
-  - Passing a type of <dfn enum-value for="XRReferenceSpaceType">bounded-floor</dfn> creates an {{XRBoundedReferenceSpace}} instance. It represents a tracking space with its [=native origin=] at the floor, where the user is expected to move within a pre-established boundary, given as the {{boundsGeometry}}. Tracking in a {{bounded-floor}} reference space is optimized for keeping the [=native origin=] and {{boundsGeometry}} stable relative to the user's environment.
+  - Passing a type of <dfn enum-value for="XRReferenceSpaceType">bounded-floor</dfn> creates an {{XRBoundedReferenceSpace}} instance. It represents a tracking space with its [=native origin=] at the floor, defined similarly to that of {{XRReferenceSpaceType/"local-floor"}}, where the user is expected to move within a pre-established boundary, given as the {{boundsGeometry}}. Tracking in a {{bounded-floor}} reference space is optimized for keeping the [=native origin=] and {{boundsGeometry}} stable relative to the user's environment.
 
   - Passing a type of <dfn enum-value for="XRReferenceSpaceType">unbounded</dfn> creates an {{XRReferenceSpace}} instance. It represents a tracking space where the user is expected to move freely around their environment, potentially even long distances from their starting point. Tracking in an {{unbounded}} reference space is optimized for stability around the user's current position, and as such the [=native origin=] may drift over time.