The simplest loader I’ve found, using only HTML/CSS. Credit to design.dev!

<div class="loading"></div>

.loading {
	width: 2rem;
	height: 2rem;
	border: .25rem solid #eee;
	border-top-color: #39f;
	border-radius: 50%;
	animation: spin 1s linear infinite;
}

@keyframes spin {
	100% {
		transform: rotate(360deg);
	}
}

What it looks like: