// note the following should be called before before calling either RTCPeerConnection.createOffer() or createAnswer()
let tcvr = pc.getTransceivers()[0];
let codecs = RTCRtpReceiver.getCapabilities('video').codecs;
let vp9_codecs = [];
// iterate over supported codecs and pull out the codecs we want
for(let i = 0; i < codecs.length; i++)
{
   if(codecs[i].mimeType == "video/VP9")
   {
      vp9_codecs.push(codecs[i]);
   }
}
// currently not all browsers support setCodecPreferences
if(tcvr.setCodecPreferences != undefined)
{
   tcvr.setCodecPreferences(vp9_codecs);
}