Google\'s "Report a Bug" or "Feedback Tool" lets you select an area of your browser window to create a screenshot that is submitted with your feedback ab
Your web app can now take a 'native' screenshot of the client's entire desktop using getUserMedia()
:
Have a look at this example:
https://www.webrtc-experiment.com/Pluginfree-Screen-Sharing/
The client will have to be using chrome (for now) and will need to enable screen capture support under chrome://flags.