If you want to add a responsive YouTube video to your Article page, follow these steps.
All responsive CSS is taken care of by the Joostrap template.
You simply have to go to YouTube and find the video you want to embed in your Joomla Article.
Then click on the Share button, then the Embed button. Copy the code as seen below, into the source code of your Joomla Article.
<iframe src="http://www.youtube.com/embed/jLHGnvnw-gI" frameborder="0"> </iframe>