2.1.1 Define the word space to suit the size and natural letterfit of the font

If text is set ragged right, the word space (the space between words) can be fixed and unchanging. If the text is justified (set flush left and right), the word space must be elastic. In either case the size of the ideal word space varies from one circumstance to another, depending on factors such as letterfit, type color, and size. A loosely fitted or bold face will need a larger interval between the words. At larger sizes, when letterfit is tightened, the spacing of words can be tightened as well.”

In CSS, word space is set with the word-spacing property which has a default value of normal. The default word space is equivalent to approximately 0.25 em, although the exact value will depend on information encoded within the digital font file. To change the word spacing, you should specify a length in ems. This length is added to the existing word space; that is to say word-spacing does not set the actual space between words, it sets an increment to the existing spacing. For example:

p {
  word-spacing:0.25em }
h1 {
  word-spacing:-0.125em }

In the preceding example, word space in paragraphs is increased by 0.25 em (effectively doubling it) and the word space in top level headings is decreased by 0.125 em (effectively halving it).

While in theory you could specify word-spacing in pixels or any other allowable unit of length, it is important to specify word-spacing in ems as that is the only way to guarantee the word space changes proportionately to the text size (when readers change their default text size, for example).

An explanation of ems

Ems are so-called because they are thought to approximate the size of an uppercase letter M (and so are pronounced emm), although 1 em is actually significantly larger than this. Bringhurst describes the em thus:

[t]he em is a sliding measure. One em is a distance equal to the type size. In 6 point type, an em is 6 points; in 12 point type an em is 12 points and in 60 point type an em is 60 points. Thus a one em space is proportionately the same in any size.

To illustrate this principle in terms of CSS, consider these styles:

#box1 {
  font-size:12px;
  width:1em;
  height:1em;
  border:1px solid black }

#box2 {
  font-size:60px;
  width:1em;
  height:1em;
  border:1px solid black }

These styles will render like:

M
M

Note that both boxes have a height and width of 1 em but because they have different font sizes, one box is bigger than the other. Box 1 has a font-size of 12 px so its width and height is also 12 px; similarly the text of box 2 is set to 60 px and so its width and height are also 60 px.

Rhythm & Proportion

§ Horizontal Motion

Reference

Cover of ‘The Elements of Typographic Style’ by Robert Bringhurst