[css3-text] letter-spacing and ligatures

> When the effective letter-spacing between two characters is not zero
> (due to either justification or a non-zero specified optimum), user
> agents should not apply optional ligatures. 

This is left over from the definition of letter-spacing with min/max values.
When letter-spacing is not 'normal', ligatures should be disabled.  They are
not disabled based on justification.

I would suggest this wording:

  When 'letter-spacing' is set to an explicit length value,
  optional ligatures enabled by default are explicitly disabled.

Cheers,

John Daggett