问组件呈现时加载组件css

EN

import React from 'react'
import './A.css'

export default class A extends React.Component {
   ....
} 

import React from 'react'
import './B.css'

export default class B extends React.Component {
   ....
}

import React from 'react'
import ReactDOM from 'react-dom'
import { Route, Switch, BrowserRouter } from 'react-router-dom'
import A from './A'
import B from './B'

ReactDOM.render(
  <BrowserRouter>
    <Switch>
        <Route exact path='/A' component={A}/>
        <Route exact path='/B' component={B}/>
      </Switch>
  </BrowserRouter>,
  document.getElementById('root'));