Converter.php

<?php

namespace CommonMark;

use CommonMark\Inline\Processor\YouTubeVideoProcessor;
use CommonMark\Inline\Renderer\YouTubeVideoRenderer;

use League\CommonMark\Converter;
use League\CommonMark\Environment;
use League\CommonMark\DocParser;
use League\CommonMark\HtmlRenderer;

class CommonMarkConverter extends Converter {
	public function __construct(array $config = []){
		$environment = new Environment($config);
		$environment->addExtension(new Extension\CommonMarkExtension());
		$environment->addInlineProcessor(new YouTubeVideoProcessor());
		$environment->addInlineRenderer(
			"CommonMark\Inline\Element\YouTubeVideo",
			new YouTubeVideoRenderer()
		);

		parent::__construct(
			new DocParser($environment),
			new HtmlRenderer($environment)
		);
	}
}

style.css

.youtube-video {
	display:block;
	position: relative;
	padding-bottom: 56.25%; /* 16:9 */
	padding-top: 25px;
	height: 0;

	> iframe {
		position: absolute;
		top: 0;
		left: 0;
		width: 100%;
		height: 100%;
	}
}

YouTubeVideo.php

<?php

namespace CommonMark\Inline\Element;

class YouTubeVideo extends \League\CommonMark\Inline\Element\Link {}

YouTubeVideoProcessor.php

<?php

namespace CommonMark\Inline\Processor;

use CommonMark\Inline\Element\YouTubeVideo;

use League\CommonMark\Inline\Processor\InlineProcessorInterface;
use League\CommonMark\Delimiter\Delimiter;
use League\CommonMark\Delimiter\DelimiterStack;
use League\CommonMark\Inline\Element\Link;
use League\CommonMark\Util\ArrayCollection;

class YouTubeVideoProcessor implements InlineProcessorInterface
{
	public function processInlines(ArrayCollection $inlines, DelimiterStack $delimiterStack, Delimiter $stackBottom = null) {
		foreach ($inlines as $key => $inline) {
			if (!($inline instanceof Link)) {
				continue;
			}

			$matched = preg_match(
				'%^https://(?:www\.youtube\.com/watch\?v=|youtu\.be/)([^&#]+)%',
				$inline->getUrl(),
				$matches
			);
			if (!$matched) {
				continue;
			}

			$inlines->set($key, new YouTubeVideo(
				"https://youtu.be/$matches[1]",
				$inline->getChildren(),
				isset($inline->data['title']) ? $inline->data['title'] : null
			));
		}
	}
}

YouTubeVideoRenderer.php

<?php

namespace CommonMark\Inline\Renderer;

use CommonMark\Inline\Element\YouTubeVideo;

use League\CommonMark\HtmlElement;
use League\CommonMark\HtmlRendererInterface;
use League\CommonMark\Inline\Element\AbstractInline;
use League\CommonMark\Inline\Renderer\LinkRenderer;

class YouTubeVideoRenderer extends LinkRenderer {
	public function render(
		AbstractInline $inline,
		HtmlRendererInterface $htmlRenderer
	){
		if (!($inline instanceof YouTubeVideo)) {
				throw new \InvalidArgumentException('Incompatible inline type: ' . get_class($inline));
		}

		$matched = preg_match(
			'%^https://(?:www\.youtube\.com/watch\?v=|youtu\.be/)([^&#]+)%',
			$inline->getUrl(),
			$matches
		);
		if(!$matched){
			return parent::render($inline, $htmlRenderer);
		}

		if (count($inline->getParent()->getInlines()) > 1) {
			return parent::render($inline, $htmlRenderer);
		}

		$iframe = new HtmlElement('iframe', [
			'width' => 640,
			'height' => 390,
			'src' => "https://www.youtube.com/embed/$matches[1]",
			'type' => "text/html",
			'frameborder' => 0,
		]);

		$div = new HtmlElement('span', ['class' => 'youtube-video'], $iframe);

		return $div;
	}
}