A shortcode to add oEmbeds that are fluid and 16:9 aspect ratio

fluid-oembeds.css

.fluid-oembed {
	position: relative;
	padding-bottom: 56.25%;
	padding-top: 30px;
	height: 0;
	overflow: hidden;
}
.fluid-oembed iframe,
.fluid-oembed object,
.fluid-oembed embed {
	position: absolute;
	top: 0;
	left: 0;
	width: 100%;
	height: 100%;
}

fluid-oembeds.php

// Add Shortcode
function fluid_oembed_shortcode( $atts ) {
	// Attributes
	extract(shortcode_atts(
		array(
			'url' => '',
		),
		$atts
  ));
  echo '<div class="fluid-oembed">' . wp_oembed_get($url) . '</div>';
}
add_shortcode( 'fluid_oembed', 'fluid_oembed_shortcode' );