Justified text

Justification is a trade, not a setting. The line has to arrive at the right margin exactly, and the only thing available to stretch is the word spaces, so every line ends up with spaces of a different width from its neighbours. Where a line happens to hold few words, or one very long one, those spaces open up into gaps, and gaps that line up down the column read as pale vertical channels running through the text. Typographers call them rivers, and once you can see them you cannot unsee them.

Print solves this with hyphenation and with judgement. A hyphenation dictionary gives the line breaker more places to break, so it can take a fragment of a long word down to the next line rather than stretching four spaces to cover it, and a good engine considers the whole paragraph at once, choosing the set of breaks with the lowest total badness rather than greedily filling one line at a time. That is what Knuth and Plass wrote for TeX, and it is why a justified book page looks calm and a justified web page usually does not.

Browsers are getting closer but they are not there. text-align: justify is a greedy line breaker, so it commits to each line without knowing what the next one costs, and hyphens: auto needs the language declared (a lang attribute the browser has a dictionary for) before it will break a word at all. Without hyphenation, justification at a narrow measure is the worst case possible: fewer words per line means each space carries more of the stretch. text-wrap: pretty improves the rag and the last line, and text-justify: inter-character exists for scripts that need it, but the missing piece is still whole-paragraph optimisation.

So the practical rule: justify only with hyphenation on, only at a comfortable measure, and prefer flush left (ragged right) everywhere else, which is what almost every interface should do. Flush left keeps one word space throughout, and a rag the eye uses to keep its place. Justified text also interacts badly with things a UI cannot control: a long URL, a proper noun that cannot be broken, or a reader who has zoomed, each of which turns one line into a row of islands.