If you run the tabs example from https://github.com/coonsta/Web-Components-Polyfill in samples/tabs/index.html the styles in the Shadow DOM are applied differently depending on whether the Enable <style scoped> flag is enabled or not. I believe <style scoped> is behind a flag because of questions about the HTML spec. I don’t think scoping styles for Web Components needs to block on that.
Oops. Dominic filed this ahead of me. I'm sorry for my overlooking. *** This bug has been marked as a duplicate of bug 95664 ***