We will need some machinery to support XBL scoped stylesheets to make video controls work. That work is tracked in bug 1290276. Once we have that, we also need to consider <style scoped>. There's discussion about removing it in bug 1291515, with people leaning to keep it. If we keep it, we'll need to implement it, which hopefully won't be too much work after bug 1290276 is done.
Over to Ting-Yu to work on alongside bug 1290276.