Per <http://www.w3.org/TR/2007/WD-css3-text-20070306/#letter-spacing>, "UAs may apply letter-spacing to cursive scripts. In this case, UAs should extend the space between disjoint graphemes as specified above and extend the visible connection between cursively connected graphemes by the same amount (rather than leaving a gap). The UA may use glyph substitution or other font capabilities to spread out the letters. If the UA cannot expand a cursive script without breaking the cursive connections, it should not apply letter-spacing between grapheme clusters of that script at all". Currently WebKit cannot expand cursive scripts without breaking the cursive connections, so it should not apply letter-spacing at all.
<rdar://problem/5698786>
How do we know if a script is cursive?