4. If getContext() was invoked with a second argument, let options be that argument.  Otherwise, let options be an empty dictionary.
5. Convert options to an IDL value of type WebGLContextAttributesInit.  If the conversion threw an uncaught exception, then abort this algorithm, with that exception being passed through to the caller.
6. For each dictionary member defined in options, find the attribute on contextAttributes whose identifier matches the key of the dictionary member, and then set the attribute to the value of that dictionary member.