Transform centering

Vertically and horizontally centers a child element within its parent element using position: absolute and transform: translate() (as an alternative to flexbox or display: table). Similar to flexbox, this method does not require you to know the height or width of your parent or child so it is ideal for responsive applications.

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

.parent {
	border: 1px solid #333;
	height: 250px;
	position: relative;
	width: 250px;
}

.child {
	left: 50%;
	position: absolute;
	top: 50%;
	transform: translate(-50%, -50%);
	text-align: center;
}

Demo

See the pen on CodePen.

Explanation

position: absolute on the child element allows it to be positioned based on its containing block.
left: 50% and top: 50% offsets the child 50% from the left and top edge of its containing block.
transform: translate(-50%, -50%) allows the height and width of the child element to be negated so that it is vertically and horizontally centered.

Note: Fixed height and width on parent element is for the demo only.