813.4.a. Those conditions might only be fulfilled while the card or ability is on the chain. In such a case, it can still be played or activated at the appropriate timing as long as doing so could fulfill the conditions.
813.4.b. If the chain item does not fulfill the conditions by the time step 5: check legality has been reached, the actions taken while playing or activating the chain item are undone and it is returned to the zone it was played from if it is a card.