Hello dan72205,
Welcome to Microsoft Community.
I understand your situation. From your description, it seems you are encountering issues when sharing your screen or video stream. There are a few details I need to confirm with you. When screen sharing and playing videos in the browser, is it that all situations fail to display on the projected screen, or is it only specific instances? For example, do only certain sites not work? If you have only tried a few websites, please try more, including video player clients. If only specific sites do not work, please provide the specific websites so I can try them.
Besides the source of the video, there are also details about the video itself, such as window mode and full-screen mode. How is the video stream not displaying? Suppose you are playing in a browser window; does it show the entire computer screen, but the video stream area only shows a black screen?
Here is what you need to do: try different sources (if it is specific sites, provide the links, as some service providers may restrict video distribution through technical means), use a player, and describe the specific manifestation of the video not displaying in the shared screen. Thank you very much for your cooperation.
Best wishes
Leo Z | Microsoft Community Support Specialist