html not rendering into dom in reactjs

I am new to ReactJS. This is my first code where I am trying to render two components. I am not getting any error but HTML part is not coming up. DOM is empty. What is wrong here? --

import React from 'react';
import ReactDOM from 'react-dom';

class First extends React.Component{
    render(){
        return (
            <h1>it is about how to render two component. it is first component</h1>
        )
    }
}

class Second extends React.Component{
    render(){
        return (
            <h1>it is about how to render two component. it is second component</h1>
        )
    }
}

class all extends React.Component{
    render(){
        return(
            <section><First /><Second /></section>
        )
    }
}

ReactDOM.render(<all />,document.getElementById('root'));