1. add a new function --- let's call it getDeviceAdvisories() --- that can be called without a WebGL context, as it will be meant to be called before getContext. For example, it could be a method on the window object.
2. getDeviceAdvisories() returns a JS Object, which may be null. By default, the browser will return a null object, which should be interpreted as "nothing special to say about this device".
* The default is to expose no information.
I propose using URIs for capability profile predicates.