Source

File: wp-includes/media.php

function wpview_media_sandbox_styles() {
	$version        = 'ver=' . get_bloginfo( 'version' );
	$mediaelement   = includes_url( "js/mediaelement/mediaelementplayer-legacy.min.css?$version" );
	$wpmediaelement = includes_url( "js/mediaelement/wp-mediaelement.css?$version" );

	return array( $mediaelement, $wpmediaelement );
}