Markup: templates/output/04-elements/videos/youtube.html
<a class="lightbox lightbox-video" href="https://www.youtube.com/embed/RK1K2bCg4J8" data-type="iframe" data-group="iframe-vimeo" data-width="1120px" data-height="630px"><img src="https://placeholder.in2code.de/800x525" width="900" height="600" alt="Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor. Aenean massa."/></a>
Source:
assets/sass/04-elements/_elements.videos.scss, line 53