Vertically align text to the bottom of the box?

2020 Update

You can use CSS grid, flexbox or the original method with line-height:

body { display: flex } /* just to prettify */

div {
  margin: .5em;
  width: 6.25em; height: 6.25em;
  background: #eee;
  color: #333;
  text-align: center
}

.grid {
  display: grid;
  align-content: end;
}

.flex {
  display: flex;
  align-items: flex-end;
  justify-content: center
}

.lh { line-height: 11.5 /* 6.25*2 - 1.5 */ }

<div class='grid'>Hello</div>

<div class='flex'>Hello</div>

<div class='lh'>Hello</div>

Setting the height of the div and the line-height of the text to the same value, 100px in your case, is a method of vertically centering the text within the div. That's the problem.

The solution is to change line-height to twice the height minus the size of the text and remove useless vertical-align.

You could set display to table-cell, try this CSS for example.

width: 100px; 
height: 100px; 
display: table-cell; 
vertical-align: bottom;

Demo: http://jsfiddle.net/Kawwr/

Enclose the text in a p tag with display:inline-block. Set vertical-align to the p element.

<div>
    <p>FoxRox</p>
</div>​

div {
    width: 100px;
    height: 100px;
    background: #eee;
    color: #333;
    text-align: center;
 }
p {
    display: inline-block;
    vertical-align: -80px;
}
​

Demo