Allows an element to preserve the x-height (measured in exes) of a "first choice" font when substituting fonts. The z-factor is a ratio of the em- to x-heights of a font. Because different fonts set to the same font size can look larger or smaller than neighboring fonts on a page set to the same size, the z-factor can be used to calculate the ratio and apply it to other fonts. Even though the resulting font size may be larger or smaller than the "first choice" font setting, the perceived size is much more accurate. This also tends to equalize the horizontal metrics of fonts so that word-wrapped lines break at the same place with different font families.