This plugin adjusts the WordPress automatic media embeds, allowing for responsive width and height scaling when the device or browser resolution changes.

Responsive WordPress oEmbeds

<?php
/**
 * Responsive Embeds in WordPress
 *
 * Custom embed sizing for basic listing template
 */
class ResponsiveVideoEmbeds
{
	/**
	 * Setup the object
	 *
	 * Attached filters and actions to hook into WordPress
	 */
	function __construct( $options = array() ) 
	{
		add_filter('embed_oembed_html', array($this, 'modify_embed_output'), 9999, 3);
		add_filter('wp_head', array($this, 'inline_css') );
	}
	
	/**
	 * Add Embed Container
	 *
	 * Wrap the video embed in a container for scaling
	 */
	public function modify_embed_output( $html, $url, $attr ) {
		
		// Pattern for removing width and height attributes
		$attr_pattern = '/(width|height)="[0-9]*"/i';
		$whitespace_pattern = '/\s+/';
		$embed = preg_replace($attr_pattern, "", $html);
		$embed = preg_replace($whitespace_pattern, ' ', $embed); // Clean-up whitespace
		$embed = trim($embed);

		// Add container
		$html = "<p class=\"embed-container\">$embed</p>\n";
		
		return $html;
	}

	/**
	 * Embed CSS
	 *
	 * CSS needed to automatically resize embedded videos, method originally 
	 * created by Anders M. Andersen at http://amobil.se/2011/11/responsive-embeds/
	 */
	function inline_css() {
?><style type="text/css">
.embed-container {
	position:relative !important;
	padding-bottom:56.25% !important; /* 16/9 ratio */
	padding-top:30px !important; /* IE6 workaround*/
	height:0 !important;
	overflow:hidden !important;
}
.embed-container iframe,
.embed-container object,
.embed-container embed {
	position:absolute !important;
	top:0 !important;
	left:0 !important;
	width:100% !important;
	height:100% !important;
}
</style><?php
	}
}
$responsive_embeds = new ResponsiveVideoEmbeds();