Rocket.Chat/app/oembed/client/oembedYoutubeWidget.html

<template name="oembedYoutubeWidget">
	{{#if parsedUrl}}
		<blockquote class="message-oembed background-transparent-darker-before">
			<a href="{{url}}" rel="noopener noreferrer" target="_blank">{{parsedUrl.host}}</a>
			{{> collapseArrow collapsedMedia=collapsedMediaVar}} <br>
			{{#unless collapsed}}
				<iframe width="270" height="200" src="{{meta.twitterPlayer}}" rel="noopener noreferrer" frameborder="0" allowfullscreen></iframe><br>
				{{{meta.description}}}
			{{/unless}}
		</blockquote>
	{{/if}}
</template>