Why the first argument in the event handler is a customEvent.detail instead of a customEvent?

Hi! Would you explain to me your idea, please? Why the **first** argument in the event handler is a **customEvent.detail** instead of a **customEvent**? This solution a little bit confusing me.

See the link to [the source code](https://github.com/the-road-to-learn-react/use-custom-element/blob/82a49ce389ed99738a743b050122f158603bbb76/src/index.js#L17)
