1. It's not HTML, it's JSX, a vaguely XML-ish DSL that is compiled to JS. You can (and some people do) write the underlying JS code directly. But, obviously,

1. It's not HTML, it's JSX, a vaguely XML-ish DSL that is compiled to JS. You can (and some people do) write the underlying JS code directly. But, obviously, writing a DSL that compiles to 'React.CreateElement.Div()' is the same (at least as far as seperation of concerns) as writing 'React.CreateEleemnt.Div()' directly. 2) Best practices focused on separation of concerns, and in particular, on avoiding embedding logic into your static HTML. The canonical example to avoid historically was to put JS directly into a links href attribute, but some have suggested Angular is guilty of something equivalent with its ng-* attributes. 3) Conversely, React advocates agree 100% on separation of concerns, but feel the "nothing that looks like HTML can be in the same file as something that looks like JS" injunction is actually an example of separation of technologies.