Require that one of 'audioCapabilities' and 'videoCapabilities' be specified in the configuration passed to navigator.requestMediaKeySystemAccess(), and require that the contentType strings used as part of the entries to 'audioCapabilities' and 'videoCapabilities' include the desired codec as part of the MIME type. This item covers two bugs though only one is listed under the status section: https://crbug.com/605661, https://crbug.com/616233
Docs: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/requestMediaKeySystemAccess