Microtypography and the -bag properties

The engine exposes a handful of controls that plain CSS does not know. They all carry the -bag- prefix, cascade and inherit like normal properties, and are safe to leave in stylesheets that are also used in browsers (unknown properties are ignored there).

Font expansion

Justification usually has only the word spaces to work with. Font expansion (the hz-program idea) additionally lets the glyphs themselves stretch or shrink by a small percentage, which evens out word spacing and helps the line breaker find better breaks. It is on by default at 5%; the percentage is the maximum change of a glyph’s width:

body { -bag-font-expansion: 0%; }   /* switch it off */
body { -bag-font-expansion: 2%; }   /* more conservative than the default */

At sane values (2 to 5 percent) the distortion is invisible at reading distance; what remains visible is the calmer word spacing.

Font expansion only acts on justified text (text-align: justify). Ragged text, which includes the default text-align: start, is set at its natural width: neither the glyphs nor the word spaces are squeezed, and a word that does not fit moves to the next line.

Line breaker tuning

Two knobs steer the paragraph breaker directly:

blockquote { -bag-linebreak-tolerance: 8; }
th         { -bag-linebreak-hyphen-penalty: 200; }

• -bag-linebreak-tolerance (default 4) is how far the spaces of a line may stretch, as a multiple of their stretchability, before the breaker refuses the line. It is a ratio, not a badness as TeX’s \tolerance: TeX’s 200 is about 1.26. Raise it for very narrow measures (table cells, margin columns) where the default finds no acceptable break and lines come out overfull; the price is looser word spacing. A value below 1 also limits shrinking: the spaces may then shrink by at most that fraction of their shrinkability.
• -bag-linebreak-hyphen-penalty makes hyphenated line ends more expensive. Raise it to hyphenate less without forbidding hyphenation outright (hyphens: none does that); it is the gentler tool because emergencies can still hyphenate. A value of 10000 or more forbids breaks at hyphens altogether, soft hyphens included.

Reference: all -bag properties