I was thinking that it's so weird to talk about using standard HTML validation and then everything is shown with React? If we want to teach people how standard works, we can't assume React as the default.
I addressed this elsewhere in the comment section, but there's not much "react" going on in the article. I do think that JSX is very expressive and the concern I cover mostly involves the declarative "component" model for writing UIs. It's not react-specific.
I believe you could probably do most of that with just css now too. Disabling the default pop up from showing up may be the only thing you need javascript to do.