wordSpacing property
Word spacing (Tw, ยง9.3.3) in em units - the extra advance added after each single-byte space (code 32) on a simple font; 0 for composite (CID) fonts, which Tw never touches. Like letterSpacing, a substituting device should apply it as real spacing so a space carrying a large Tw opens a genuine gap rather than stretching the surrounding glyphs across it.
Implementation
final double wordSpacing;