Constraints and MediaRecorder

I'm skeptical of the Constraints API in general, but I think MediaRecorder
at least doesn't seem to need it.

Suppose we had a MediaRecorderOptions dictionary like so:
dictionary MediaRecorderOptions {
  DOMString mimeType;
  unsigned long width;
  unsigned long height;
};
and pass it as an additional argument to the constructor. Further suppose
we specify that any video frames are aspect-preserving-scaled to fit within
width x height if both are specified, or if only one is specified then the
video frames are aspect-preserving-scaled to that dimension. The mimeType
is used if supported, otherwise the UA ignores it and makes its own choice.

What are the use-cases that approach fails to solve? Maybe there are some,
but the usability penalty going from
var mr = new MediaRecorder(stream, {mimeType:"audio/opus"});
to
var mr = new MediaRecorder(stream);
mr.applyConstraints({optional:[{mimeType:"audio/opus"}]}, function
onsuccess() { ... });
(in particular going from sync to async) is annoying. Not to mention the
spec and WebIDL issues of indirecting through Constraints and pulling in
that machinery.

Rob
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