Currently, getCapabilities is defined as follows:

partial interface RTCRtpSender {
static RTCRtpCapabilities getCapabilities ();
};

partial interface RTCRtpReceiver {
static RTCRtpCapabilities getCapabilities ();
};

The implication is that getCapabilities() returns both audio and video capabilities in a single RTCRtpCapabilities object.

However, if getCapabilities wasn't defined as static, then it could return only capabilities of the appropriate kind:

For example, with respect to the RTCRtpSender object, it could work this way:

var audioSender = new RTCRtpSender(audioTrack, transport);
var videoSender = new RTCRtpSender(videoTrack, transport);
rtpAudioSendCaps = audioSender.getCapabilities();
rtpVideoSendCaps = videoSender.getCapabilities();

With the RTCRtpReceiver object, it could work this way:

   var audioReceiver = new RTCRtpReceiver(transport);
   var videoReceiver = new RTCRtpReceiver(transport);
   rtpAudioRecvCaps = audioReceiver.getCapabilities("audio"); 
   rtpVideoRecvCaps = videoReceiver.getCapabilities("video"); 

The advantage of doing it this way is that a subsequent call to createParameters could be more specific:

var audioRecvParams = RTCRtpReceiver.createParameters(
"audio", remote.rtpAudioSendCaps);
var videoRecvParams = RTCRtpReceiver.createParameters(
"video", remote.rtpVideoSendCaps);

So an alternative would be as follows:

partial interface RTCRtpReceiver {
RTCRtpCapabilities getCapabilities (DOMString kind);
}

partial interface RTCRtpSender {
RTCRtpCapabilities getCapabilities ();
};