To enable screen sharing on the latest Mac version in the browser (Chrome in this example), you need to set the following system preferences:
You navigate to security from the system preferences:
In the Security panel, navigate to Privacy and enable screen sharing for the Chrome browser by checking the Google Chrome box. Note that you need to click on the lock in the bottom left corner before you can check the box.
For other browsers except Safari, it works the same way. For Safari, as soon as you get the question if you want to share your example, you have to click "Allow".