You can try this basic approach:

div {
  height: 100px;
  line-height: 100px;
  text-align: center;
  border: 2px dashed #f69c55;
}

<div>
  Hello World!
</div>

It only works for a single line of text though, because we set the line's height to the same height as the containing box element.

A more versatile approach

This is another way to align text vertically. This solution will work for a single line and multiple lines of text, but it still requires a fixed height container:

div {
  height: 100px;
  line-height: 100px;
  text-align: center;
  border: 2px dashed #f69c55;
}
span {
  display: inline-block;
  vertical-align: middle;
  line-height: normal;
}

<div>
  <span>Hello World!</span>
</div>

The CSS just sizes the <div>, vertically center aligns the <span> by setting the <div>'s line-height equal to its height, and makes the <span> an inline-block with vertical-align: middle. Then it sets the line-height back to normal for the <span>, so its contents will flow naturally inside the block.