2.1.8 Kern consistently and modestly or not at all

Kerning – altering the space between selected pairs of letters – can increase consistency of spacing in a word like Washington or Toronto, where the combinations Wa and To are kerned.”

At present there is no mechanism within HTML or CSS that specifically enables kerning. However text on the Web does not generally need manual kerning, as all digital fonts have kerning tables built-in. These tables define which letter pairs need adjusting and by how much, and are usually adhered to by operating systems. The only time manual kerning may prove necessary is with larger text such as that in headings, in particular when numbers, italics or punctuation are involved.

It is also important to bear in mind that letter pairs in one font of your font-family list may need kerning whereas in another font they do not. In this case it is advisable not to kern, as too much kerning is almost always worse than none at all.

Where you do wish to kern letter pairs, you must insert a neutral inline element, such as span and apply the letter-spacing property. For example:

<span class="kern">W</span>ashington
and <span class="kern">T</span>oronto

.kern { letter-spacing: -0.1em; }

Which should render as:

Washington and Toronto

Notice that the span was applied around the first letter of the pair, not around both letters. This is because the letter-spacing property adds or removes space after each letter. Notice also that the letter-spacing is specified in ems to ensure that the amount of kerning is applied in proportion to the text size.

As with letterspacing strings of capitals, adding a span every time you need to kern a letter pair could become tedious and so regular expressions could come to the rescue once more. This PHP function uses a regular expression to replace insert a span into any string of ‘To’ or ‘Wa’:

$search = '/(T)(o)|(W)(a)/';
$replace = '<span class="kern">$1$3</span>$2$4';

The Future

The CSS3 Text Module may contain the kerning-mode and kerning-pair-threshold properties to aid control over kerning, although these properties have yet to be fully defined.

Rhythm & Proportion

§ Horizontal Motion


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