When Yasuo Remorseful attacks, can the opponent play any actions before taking damage from the Yasuo attack trigger, or does the showdown begin after the attack defend triggers?

According to the rules and FAQs for Yasuo, Remorseful, here is the ruling on your question:

Ruling: When Yasuo, Remorseful moves to an occupied battlefield, the Showdown begins immediately with an Initial Chain. During this Initial Chain, only Reaction speed cards can be played. You cannot play Action speed cards until the Initial Chain has fully resolved and the pile is free.

Sequence of Events:

1. Yasuo, Remorseful moves to an occupied battlefield, initiating a Showdown.
2. The Showdown begins with an Initial Chain.
3. Yasuo's "When I attack" trigger is placed on the Initial Chain.
4. Only Reaction speed cards can be played in response to this trigger. Action speed cards cannot be played at this time.
5. Once the Initial Chain resolves (and the stack is empty), the state becomes Open.
6. Only after the Initial Chain resolves does the attacker gain Focus and the opportunity to play Action speed cards.

Summary: The opponent cannot play Actions before the Initial Chain resolves. They must wait until the Initial Chain is empty and the attacker has gained Focus to play Action speed cards.

Reference: This ruling is based on the provided FAQ which states: "Only Reactions can be played in response to the Initial Chain; Actions cannot be played until after the Initial Chain resolves and the attacker gains Focus."