Word spacing

The gap between two words is a character, and like every other character in the font it was drawn by somebody. Its width is part of the face’s fit, typically somewhere near a quarter of an em, and it is tuned against the rest of the design: a face with tight letterfitting gets a narrower space, a wide face gets a broader one. The CSS property named after it does not set that width, it adds to it, which is why word-spacing: 0.3em means three tenths of an em on top of whatever the designer already decided rather than a space three tenths of an em wide.

Most of the time the value you should add is zero. The place where the gaps move anyway is justified text, which reaches both margins by stretching the word spaces on every line until the last word lands flush, so the narrower the measure, the more each space has to give. Stretch them far enough and two things happen: the paragraph’s colour goes patchy, because white space is now unevenly distributed through the grey, and gaps on consecutive lines start falling under one another until the eye joins them into a river. The fixes are more places to break and less to stretch: hyphenation gives the line breaker somewhere else to go, and a rag removes the stretching entirely.

It is worth keeping separate from the space between letters. Widening the gaps between words separates the words; widening the gaps between letters, which is tracking, loosens the words themselves and can make them stop reading as units at all. The two failures look alike from a distance and have opposite fixes. Nearby is a much older argument about how many spaces follow a full stop, where the answer for typeset text is one: the double space is a monospace typewriter habit for a machine that could not vary the width of anything, and in a proportional face it punches holes in the paragraph. Where a space must not break at all, that is a non-breaking space rather than a spacing decision.

There is an accessibility floor under all of this. WCAG’s text spacing criterion says a reader must be able to push word spacing to at least 0.16em, along with the other spacing knobs, without losing content or function, which in practice means a layout that survives being pulled apart rather than one that was measured to the pixel. If widening the word spaces of a paragraph clips it or hides a button, the fault is in the container, not in the reader.