How do I vertically align text in a div?

The correct way to do this in modern browsers is to use Flexbox.

See this answer for details.

See below for some older ways that work in older browsers.

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Vertical Centering in CSS
http://www.jakpsatweb.cz/css/css-vertical-center-solution.html

Article summary:

For a CSS 2 browser, one can use display:table/display:table-cell to center content.

A sample is also available at JSFiddle:

div { border:1px solid green;}

<div style="display: table; height: 400px; overflow: hidden;">
  <div style="display: table-cell; vertical-align: middle;">
    <div>
      everything is vertically centered in modern IE8+ and others.
    </div>
  </div>
</div>

Expand snippet

It is possible to merge hacks for old browsers (Internet Explorer 6/7) into styles with using # to hide styles from newer browsers:

div { border:1px solid green;}

<div style="display: table; height: 400px; #position: relative; overflow: hidden;">
  <div style=
    "#position: absolute; #top: 50%;display: table-cell; vertical-align: middle;">
    <div style=" #position: relative; #top: -50%">
      everything is vertically centered
    </div>
  </div>
</div>