Go to http://jonsuh.com/blog/responsive-youtube-vimeo-embed-and-html5-video-with-css/ to get more information.

More and more people are accessing the internet with mobile devices, it’s so important that you give them best mobile experience when they visit your website. Today, I have a quick and easy tutorial of how to make YouTube, Vimeo, embed, and HTML5 videos responsive with CSS.

Resize your browser, view this page on your mobile device, or change your device orientation (landscape, portrait). You’ll see that the video resizes proportionally, not to mention the rest of this site, according to the size of your browser or orientation of your device.

The HTML is extremely simple:

Simply wrap the iframe, embed, or video tags with a div with class js-video. The optional .widescreen gives support for widescreen 16:9 videos and .vimeo tweaks any Vimeo videos you embed for the best experience.