When we're trying to extend the mock-up space we always have to scan on DOM that on the one hand people want to style everything, everybody wants to to change the controls of the video with CSS, on the other hand we have people who built widgets who want to invent them in the page and not allow people to style them, not allowed to styling of the main page to seep through and change the background color to pink unicorns that you don't necessarily want to have in your Linkedln plugin or something 