The theory and the application of optimal line height — called leading in typographic circles — varies greatly in web design. Typography references consistently put ideal line height at 1.2 ems (a measure of type equivalent to the the letter height or point size of a typeface).
Yet, premier web designers (and typography affectionatos) regularly set line height to 1.5 ems or more. That is a 33% overstatement of the preferred height. This creates a striping of text, rather than the desired greying. When Jason Santa Maria crafted the current A List Apart design, he set the line height to a whopping 1.8 ems. Why is this done?
This is partly explained when one considers the line length of the subject designs. The designs require line lengths (or measure) notably longer than those recommended in typographic references. The increased line length demands increased line height to aide eye tracking. This does not make for ideal type, but it does make for the best typography for the subject design. James Felici says it well in his book The Complete Manual of Typography:
An axiom of computerized type is that leading should generally be 1.2 times the point size of type. This isn’t a terribly useful guideline, because it’s clear that leading must increase along with the measure.
One consequence of this is other web designers look at what the élite are doing, and apply that line height regardless of line length. This leads to line heights that are unnecessarily out of proportion.
So as a general rule, an optimal line length is 60 to 65 characters, or 30 ems. At that measure, the ideal line height should be approximately 1.2 ems. As line length increases, line height should follow proportionately. The body text in the KINGdesk design is set to a measure of nearly 27 em and a line height of 1.2 ems.