Webrtc screencapture or capturestream

Is there a way to capture on a specific part of the screen.. getting only the element or tag of HTML… I just want to share the video that is playing. I tried to find a solution. I tried the capturestream but the problem is there is no Output on my client-side. Any suggestion or who tried to stream only the video payer on capturestream?

Anyone who successfully broadcast live of the capturestream with broadcaster and client?