example.jsx

export class MyComponent extends React.Component {
  componentWillMount() {
    const styleid = 'MyComponentStyles'
    if (document.head.querySelector(`#${styleid}`) !== null) {
      const styles = document.createElement('style');
      styles.id = styleid;
      styles.append('[foo]{ color: blue; }');
      document.head.appendChild(styles);
    }
  }

  render() {
    return (
      <div foo>
        <p>Awesome</p>
      </div>
    );
  }
}