Defer loading of YouTube player

defer-youtube-load-with-thumbnail.php

<?php

add_shortcode( 'pavenum_video', function( $atts ) {

    $a = shortcode_atts( array(
        'id' => '',
        'ratio' => '',
        /*'legend' => '',*/
        'alt' => ''
    ), $atts );

$html = '<style>.youtube {
	position: relative;
	padding-top: 56.23%;
	overflow: hidden;
	cursor: pointer;
    background-color: black;
	border-radius: var(--frame-radius);
}
.youtube img {
	width: 100%;
    height: 100%;
    top: 0;
	left: 0;
    object-fit: cover;
    opacity: 0.5;
}
.youtube .play-button {
	width: 90px;
	height: 60px;
	background-color: var(--secondary);
	box-shadow: var(--box-shadow);
	z-index: 1;
	border-radius: 6px;
}
.youtube .play-button:before {
	content: "";
	border-style: solid;
	border-width: 15px 0 15px 26.0px;
	border-color: transparent transparent transparent #fff;
}
.youtube img,
.youtube .play-button {
	cursor: pointer;
}
.youtube img,
.youtube iframe,
.youtube .play-button,
.youtube .play-button:before {
	position: absolute;
}
.youtube .play-button,
.youtube .play-button:before {
	top: 50%;
	left: 50%;
	transform: translate3d( -50%, -50%, 0 );
}
.youtube iframe {
	height: 100%;
	width: 100%;
	top: 0;
	left: 0;
}</style>';

$html .= '<div class="wrapper"><div class="youtube" data-embed="' . $a['id'] . '" data-alttext="' . $a['alt'] . '" data-placeholder=""><div class="play-button"></div></div></div>';

$html .= '<script>( function() {

	var youtube = document.querySelectorAll( ".youtube" );
	
	for (var i = 0; i < youtube.length; i++) {
		
		//var source = "https://img.youtube.com/vi/"+ youtube[i].dataset.embed +"/hqdefault.jpg";
        var source = "https://i.ytimg.com/vi_webp/"+ youtube[i].dataset.embed +"/maxresdefault.webp";
        //var source = youtube[i].dataset.placeholder;
        var alt_text = youtube[i].dataset.alttext;
		
		var image = new Image();
				image.src = source;
      			image.alt = alt_text;
				image.addEventListener( "load", function() {
					youtube[ i ].appendChild( image );
				}( i ) );
		
				youtube[i].addEventListener( "click", function() {

					var iframe = document.createElement( "iframe" );

							iframe.setAttribute( "frameborder", "0" );
							iframe.setAttribute( "allowfullscreen", "" );
							iframe.setAttribute( "src", "https://www.youtube.com/embed/"+ this.dataset.embed +"?rel=0&showinfo=0&autoplay=1" );

							this.innerHTML = "";
							this.appendChild( iframe );
				} );	
	};
	
} )();</script>';

return $html;

});