Source: dist/css/library_boot.css, line 9226
Example
Default styling
<iframe width="560" height="315" src="https://www.youtube.com/embed/videoseries?list=PLCROrnGF2SYByKd5KHoqjRZXM4hwlHgjH" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>