({supported: true, configuration: await navigator.requestMediaKeySystemAccess('com.widevine.alpha', [{
  initDataTypes: ['cenc'],
  videoCapabilities: [{
    contentType: 'video/mp4; codecs="avc1.42E01E"',
    robustness: 'SW_SECURE_DECODE'
  }]
}]).then(a => a.getConfiguration()).catch(() => null)})

{
    "supported": false,
    "error": "Unsupported keySystem or supportedConfigurations."
}