This CSS can be used to style lazy loaded Youtube videos without WordPress. See instructions: https://wordpress.org/support/topic/plugin-not-activating-how-to-use-plugin-with-youtube-placed-directly-in-theme?replies=4#post-6041175

Lazy Load for Youtube (CSS)

a.lazy-load-youtube {
	vertical-align: top;
	background: #555;
  width: 100%;
	max-width: 480px;
	height: 270px;
	background-size: cover;
	text-decoration: none;
	color: #ddd;
	display: inline-block;
}
a.lazy-load-youtube:hover {
  text-decoration:none;
  color: #fff;
}
.lazy-load-youtube-info {
	color: #2c2c29;
	padding: 20px 0;
	font-size: 21px;
}
.lazy-load-youtube-div {
  border: 3px solid #2c2c29;
  background: url(images/play-youtube.png) center center no-repeat;
  position: absolute;
  opacity: 0.8;
}
.lazy-load-youtube-div:hover {
  opacity: 1;
}