Can you respond to combat attack/defend triggers (specifically Yasuo's ability) with reactions?

Ruling: Yes, reactions can be played in response to combat triggers like Yasuo's ability. Reactions can be played anytime an action could be played plus in response to any trigger on the chain.

Sequence:

• Yasuo's ability triggers and requires a target to be declared immediately upon triggering
• After the target is declared, a round of priority is passed starting with the active player
• When priority passes to the opponent, they can play a reaction
• If a reaction is played, a new round of priority is passed
• When priority is passed with no responses, the Yasuo trigger resolves as much as it can

Nuances:

• Actions cannot be played in response to the ability during combat, only reactions
• The target must be declared before any player can respond with priority