Kerning

Kerning is a pair operation. It names two letters and moves them closer together or further apart, because the shapes they were drawn with leave a hole or a collision when they meet. Tracking is the other half of the pair readers mix up, and the difference is the scope: tracking adds one uniform amount to every letter in a run, while kerning is a value that applies to T next to o and to nothing else on the line. You kern a pair; you track a line.

Most kerning is already done. The type designer walks the awkward combinations and writes a correction for each into the font’s kern table, which the shaping engine applies whenever those two letters turn up together, so the word “To” in a well-made face is already tightened before anyone looks at it. The pairs anyone notices are the diagonal or open shape against the round or narrow one: T over o, V beside a, Y before e, W against a comma, and capitals standing next to lowercase generally. A table typically holds a few hundred of them. What is left over is the hand kerning a designer does at poster sizes, one pair at a time, where a table tuned for text cannot know how the line will be used.

On the web the table is the whole story. font-kerning: normal asks for it, none switches it off, and auto leaves the decision to the browser, which in practice means on for the text sizes that matter. Two things quietly remove it. Shaping stops at element boundaries, so wrapping one letter of a pair in its own span to colour it also throws away the pair’s kern, and a font subset that drops the kern and GPOS data ships a face whose corrections are simply gone. Neither failure announces itself: the line just looks slightly lumpy at display size.

What no browser offers is the third way. Optical kerning computes the space from the outlines rather than reading it from the table, and since no engine implements it, web typography is the font’s pairs or nothing. That puts the weight back on the face: at 16px a poor pair costs almost nothing, and at 72px it is the first thing a reader sees, which is why display type is where both the font choice and the correction are worth the attention. Body text is better served by getting the leading right.