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.embed-wrapper{
	padding-top: 56.25%;
	position: relative;
	display:block;
	width: 100%;
	overflow: hidden;
	height: 0;
}

.embed-wrapper iframe{
	position: absolute;
	left: 0; top: 0; right: 0;
	height: 100%;
	width: 100% !important;
}

.embed-wrapper{
	padding-top: 56.25%;
	position: relative;
	display:block;
	width: 100%;
	overflow: hidden;
	height: 0;

	iframe{
		position: absolute;
		left: 0; top: 0; right: 0;
		height: 100%;
		width: 100% !important;
	}
}

<?php 
// Ajoute une div pour encadrer les iframes
add_filter('oembed_dataparse', function($return, $data, $url) {
    if ($data->provider_name == 'YouTube') {
        return "<div class='embed-wrapper'>{$return}</div>";
    } else {
        return $return;
    }
},
10,3);
?>