index.html

<!DOCTYPE html>

<html>
    <head>
        <title>My First OAuth 2 App</title>
    </head>
    <body>
        <div id='info'>
            HOI!
        </div>

        <a id="login" style="display: none;" href="your oauth2 url here">Identify Yourself</a>
        <script>
            window.onload = () => {
                const fragment = new URLSearchParams(window.location.hash.slice(1));

                if (fragment.has("access_token")) {
                    const accessToken = fragment.get("access_token");
                    const tokenType = fragment.get("token_type");

                    fetch('https://discord.com/api/users/@me', {
                        headers: {
                            authorization: `${tokenType} ${accessToken}`
                        }
                    })
                        .then(res => res.json())
                        .then(response => {
                            const { username, discriminator } = response;
                            document.getElementById('info').innerText += ` ${username}#${discriminator}`;
                        })
                        .catch(console.error);

                }
                else {
                    document.getElementById('login').style.display = 'block';
                }
            }
        </script>
    </body>
</html>