Gate xrCompatible on permissions policy

Author: Manishearth

index.bs

@@ -2076,7 +2076,8 @@ When a user agent implements this specification it MUST set a <dfn>XR compatible
 
 Note: This flag introduces slow synchronous behavior and is discouraged. Consider calling {{WebGLRenderingContextBase/makeXRCompatible()}} instead for an asynchronous solution.
 
-The [=XR compatible=] boolean can be set either at context creation time or after context creation, potentially incurring a context loss. To set the [=XR compatible=] boolean at context creation time, the {{xrCompatible}} context creation attribute must be set to <code>true</code> when requesting a WebGL context.
+The [=XR compatible=] boolean can be set either at context creation time or after context creation, potentially incurring a context loss. To set the [=XR compatible=] boolean at context creation time, the {{xrCompatible}} context creation attribute must be set to <code>true</code> when requesting a WebGL context.   If the requesting document's origin is not allowed to use the "xr-spatial-tracking" [[#permissions-policy|permissions policy]], {{xrCompatible}} has no effect.
+
 
 The {{WebGLContextAttributes/xrCompatible}} flag on {{WebGLContextAttributes}}, if <code>true</code>, affects context creation by requesting the user-agent [=create the WebGL context=] using a [=compatible graphics adapter=] for the [=immersive XR device=]. If the user agent succeeds in this, the created context's [=XR compatible=] boolean will be set to true. To obtain the [=immersive XR device=], [=ensure an immersive XR device is selected=] SHOULD be called.