How do I vertically center text with CSS?

You can easily do this by adding the following piece of CSS code:

display: table-cell;
vertical-align: middle;

That means your CSS finally looks like:

#box {
  height: 90px;
  width: 270px;
  background: #000;
  font-size: 48px;
  font-style: oblique;
  color: #FFF;
  text-align: center;
  margin-top: 20px;
  margin-left: 5px;
  display: table-cell;
  vertical-align: middle;
}

<div id="box">
  Some text
</div>

Another way (not mentioned here yet) is with Flexbox.

Just add the following code to the container element:

display: flex;
justify-content: center; /* align horizontal */
align-items: center; /* align vertical */

Flexbox demo 1

.box {
  height: 150px;
  width: 400px;
  background: #000;
  font-size: 24px;
  font-style: oblique;
  color: #FFF;
  text-align: center;
  padding: 0 20px;
  margin: 20px;
  display: flex;
  justify-content: center;
  /* align horizontal */
  align-items: center;
  /* align vertical */
}

<div class="box">
  Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam nonummy nibh
</div>

Alternatively, instead of aligning the content via the container, flexbox can also center a flex item with an auto margin when there is only one flex-item in the flex container (like the example given in the question above).

So to center the flex item both horizontally and vertically just set it with margin:auto

Flexbox Demo 2

.box {
  height: 150px;
  width: 400px;
  background: #000;
  font-size: 24px;
  font-style: oblique;
  color: #FFF;
  text-align: center;
  padding: 0 20px;
  margin: 20px;
  display: flex;
}
.box span {
  margin: auto;
}

<div class="box">
  <span>margin:auto on a flex item centers it both horizontally and vertically</span> 
</div>

NB: All the above applies to centering items while laying them out in horizontal rows. This is also the default behavior, because by default the value for flex-direction is row. If, however flex-items need to be laid out in vertical columns, then flex-direction: column should be set on the container to set the main-axis as column and additionally the justify-content and align-items properties now work the other way around with justify-content: center centering vertically and align-items: center centering horizontally)

flex-direction: column demo

.box {
  height: 150px;
  width: 400px;
  background: #000;
  font-size: 18px;
  font-style: oblique;
  color: #FFF;
  display: flex;
  flex-direction: column;
  justify-content: center;
  /* vertically aligns items */
  align-items: center;
  /* horizontally aligns items */
}
p {
  margin: 5px;
  }

<div class="box">
  <p>
    When flex-direction is column...
  </p>
  <p>
    "justify-content: center" - vertically aligns
  </p>
  <p>
    "align-items: center" - horizontally aligns
  </p>
</div>

A good place to start with Flexbox to see some of its features and get syntax for maximum browser support is flexyboxes

Also, browser support nowadays is very good: caniuse

For cross-browser compatibility for display: flex and align-items, you can use the following:

display: -webkit-box;
display: -webkit-flex;
display: -moz-box;
display: -ms-flexbox;
display: flex;
-webkit-flex-align: center;
-ms-flex-align: center;
-webkit-align-items: center;
align-items: center;

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>