Although WordPress does make the process of embedding Youtube (among others) videos easy, it does not add a div around embed codes by default, so this is an essential step into making the video embeds responsive. Use the code below to add a wrapping div around each embedded video and also add the required CSS styles:

That’s all! You now have fully responsive iframe, object, embed & video (html5) elements. The responsive CSS will always show videos in 16:9 aspect ratio, which is HD.