The seekable read-only property of the HTMLMediaElement returns a TimeRanges object that contains the time ranges that the user is able to seek to, if any.
The TrackDefault() getter method of the TrackDefaultList interface allows the TrackDefault objects in the list to be accessed with an array operator (i.e. [].)
The addSourceBuffer() method of the MediaSource interface creates a new SourceBuffer of the given MIME type and adds it to the MediaSource's SourceBuffers list.