I did some experiments with WebRTC's getUserMedia() constraints? This lead me down a rat hole but I had some interesting results.

Highlights include:

• Non-obvious Findings
• Mandatory constraints are really suggestions.
• Kill the stream before applying new constraints.
• Use https for a better user experience.
• Don’t bother changing constraints in Firefox today.
• Auto-identifying Camera Resolutions

See the webrtcHacks post.