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.


Simulating table display

And here is another option, which may not work on older browsers that don't support display: table and display: table-cell (basically just Internet Explorer 7). Using CSS we simulate table behavior (since tables support vertical alignment), and the HTML is the same as the second example:

div {
  display: table;
  height: 100px;
  width: 100%;
  text-align: center;
  border: 2px dashed #f69c55;
}
span {
  display: table-cell;
  vertical-align: middle;
}

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

Using absolute positioning

This technique uses an absolutely positioned element setting top, bottom, left and right to 0. It is described in more detail in an article in Smashing Magazine, Absolute Horizontal And Vertical Centering In CSS.

div {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100px;
  width: 100%;
  border: 2px dashed #f69c55;
}

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