Define the word space to suit the size and natural letterfit of the font | The Elements of Typographic Style Applied to the Web

"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.