Previously, only the Mercury's Treads reduced the effect of any kind of CC (by 1 second if my memory is ok).
Now we have 3 items that allow you to reduce any kind of CC: Mercury's Treads, Moonflair Spellblade and Cloak and Dagger. All of these items gives you +35 Tenacity.
How does Tenacity work? Is it a kind of percent reduction based on the duration of the CC or it is a specific amount of static time for all the CCs?