Very specifically, this is not mixing "logic and markup" in the way that was taboo. This is all UI logic. Should a button be displayed or not. How large should

Very specifically, this is not mixing "logic and markup" in the way that was taboo. This is all UI logic. Should a button be displayed or not. How large should this rectangle be. React encourages you to place that logic close to where it's used so it's easier to understand. The line React devs has used is when people look at JSX and shout "separation of concerns", what they're really grasping at is "separation of languages".