now there is navigator.getUserMedia() function for html5
But JavaScript can only return 1 stream returned by getUserMedia can capture the stream by successCallback.

For example:
getUserMedia({video:true},successCallback,errorCallback)
function successCallback(stream)
{
  video.src = URL.createObjectURL(stream)
}

Now when we develop some face detection feature using html5, we have 2 or more camera connect to browser, but only by browser UI can we choose which camera can we use.We can not let JS to choose which camera can use, then use JS to manipulate the camera chosen for face detection.

So, can we in future HTML let getUserMedia return references for all available cameras connect to browser in a array, so we can iterate streams and let JS to choose the one we want for later processing, Or in some 3D scenario, to manipulate multiple streams from cameras to show some 3D scene. for example a better quality camera for face detection. 

And streams Array returned by getUserMedia can contain quality information （1 standard for 1 mega-pixel, 5 standard for mega-pixel ）or optionally the description of the camera, so JS can choose by filtering or comparision.

For example:
getUserMedia({video:true},successCallback,errorCallback)
function successCallback((Array type)streams)
{
 while(i<streams.length）
 {
 if streams(i).quality>5
 video.src=url.createObjectURL(stream(i))
 break;
 ......
 }
}