HTML

The basic HTML should look like this.

<div class="parent">
	<div class="child"></div>
</div>

Below are many different methods to center your content with CSS.

Flexbox

.parent {
	display: flex;
	align-items: center;
	justify-content: center;
}

Position

.parent {
	position: relative;
}
.child {
	position: absolute;
	top: 50%;
	left: 50%;
	transform: translate(-50%, -50%);
}

Grid

.parent {
	display: grid;
	justify-content: center;
	align-items: center;
}

Table

.parent {
	display: table;
	width: 100px;
	height: 100px;
}
.child {
	display: table-cell;
	vertical-align: middle;
	width: 50px;
	height: 50px;
	text-align: center;
}

Text align

.parent {
	display: block;
	text-align: center;
}
.child {
	display: inline-block; //key point
}

Auto margin

.child {
	width: 50%;
	margin: 0 auto;
}