Not quite standardized, but Firefox appears to allow extension-less screensharing as of FF 52

https://wiki.mozilla.org/Screensharing

https://mozilla.github.io/webrtc-landing/gum_test.html

This is all done with parameters specified to getUserMedia as opposed to Chrome's extension-only "chrome.desktopCapture.chooseDesktopMedia"

As far as I can tell, the reason why screen-sharing is not more widely accessible is because of a security UX issue rather than a technical limitation. Both Firefox and Chrome are can provide you with media stream ID's to use in WebRTC negotiation, they differ mostly in the boundaries required to authorize access.