Sometime recently (post-Sierra, but unfortunately, I can't remember exactly when) I started having problems viewing streaming video. Many video sites, including YouTube, show multiple controls -- with the Safari native player controls below the those of the site's own player. (See attached.) Sometimes I am able to get the video to play, and sometimes I can't. - I don't have Flash installed at all on this browser. - The problems continues with *all* plug-ins and all extensions disabled. - And but I have been running the latest MacOS betas, and am currently running Safari 10.1.2. - It does not happen with the Safari Technology Preview (https://developer.apple.com/safari/technology-preview/). I would welcome any suggestions about what is causing this, and how to resolve it. Thank you!