1. When an ZWNJ is used to inhibit a required ligature, (increased) letterspacing should not be affected:

    Code Zeit‌one
    Sample Zeit‌zone – Zeit‌zone
    Correct display screenshot correct Firefox 3.6.12 display of “Zeitzone”

  2. When ZWNJ or ZWJ are used in a place where they have no effect, (increased) letterspacing should not be affected:

    Code ſchl‍ieße‌n
    Sample ſchl‍ieße‌n – ſchl‍ieße‌n
    Correct display screenshot correct Firefox 3.6.12 display of “schließen”

Screenshot of the correct display habe been taken with Firefox 3.6.12 on Mac OS X.