Dva build  
 the first type: 
...
import styles from "./index.css";
function Index(){
  return(
    <div className={style.index}><div>
  )
}
export default Index;second type:
...
import "./index.css";
function Index(){
  return(
    <div className="index"><div>
  )
}
export default Index;now the first one is effective and the second is invalid. Why is the second invalid?
< hr >