I understood the concept of the offline slashing like bellow.
When the both condition #1 and #2 are set,
the nodes will be slashed.
Did not produce any block during an epoch(12 hours in CTC?)
&& Did not broadcast ImOnline(heartbeat) message during an epoch.
The number of nodes that meet condition #1 is more 20% of total validators.
Q0: Am I on the right page?
Q1: How long is the heartbeat interval?
Q2: Is it correct?
50 validators are active.
If one node produced 1 block at the begining of the era,
and went offline immediately.
Just before the end of the era,
the node begins sending the hearbeat message.
In this case, the node can evade being slashed,
also is eligible for the staking rewards.
And can be elected validator for the next era.