examples/nextjs-live-tracker-map/pages/_document.js

import Document, { Html, Head, Main, NextScript } from "next/document";

class MyDocument extends Document {
  render() {
    return (
      <Html>
        <Head>
          <link rel="icon" href="/favicon.ico" />
          <link
            rel="stylesheet"
            href="https://unpkg.com/leaflet@1.6.0/dist/leaflet.css"
            integrity="sha512-xwE/Az9zrjBIphAcBb3F6JVqxf46+CDLwfLMHloNu6KEQCAWi6HcDUbeOfBIptF7tcCzusKFjFw2yuvEpDL9wQ=="
            crossOrigin=""
          />
          <script
            src="https://unpkg.com/leaflet@1.6.0/dist/leaflet.js"
            integrity="sha512-gZwIG9x3wUXg2hdXF6+rVkLF/0Vi9U8D2Ntg4Ga5I5BZpVkVxlJWbSQtXPSiUTtC0TjtGOmxa1AJPuV0CPthew=="
            crossOrigin=""
          ></script>
        </Head>
        <body>
          <Main />
          <NextScript />
        </body>
      </Html>
    );
  }
}

export default MyDocument;