aaa.html

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <meta http-equiv="X-UA-Compatible" content="ie=edge" />
        <title>Document</title>
        <style>

            ul.slides:nth-child(2) {
                color: rebeccapurple;
                font-size: 30px;
            }

        </style>
    </head>
    <body>
        <input type="file">
        <script>
          let input = document.querySelector("input");
          input.addEventListener("change", () => {
            for (let file of Array.from(input.files)) {
                let reader = new FileReader();
                reader.addEventListener("load", () => {
                    console.log("File", file.name, "starts with",
                                reader.result);
                });
                reader.readAsText(file);
            }
          });
        </script>
    </body>
</html>