Authentication Form

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Login & Registration Form</title>
    <link rel="preconnect" href="https://fonts.googleapis.com">
    <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
    <link href="https://fonts.googleapis.com/css2?family=Noto+Sans:[email protected];500&display=swap" rel="stylesheet"> 
    <style>
        * {
            margin: 0;
            padding: 0;
            font-family: 'Noto Sans', sans-serif;
        }

        body {
            background-color: #292929;
        }

        section {
            display: flex;
            justify-content: center;
            align-items: center;
            width: 100%;
            height: 100vh;
        }

        section aside {
            background-color: #fff;
            border-radius: 5px;
            width: 350px;
            overflow: hidden;
        }

        section aside div#login,
        section aside div#reg {
            padding: 20px;
            transition: 0.5s all ease-in-out;
        }

        form h2 {
            text-align: center;
            margin-bottom: 10px;
            color: #292929;
            font-weight: 500;
            font-size: 16px;
            transition: 0.5s all ease-in-out;
            cursor: pointer;
        }

        form h2 small {
            font-size: 12px;
            color: #f00;
            transition: 0.5s all ease-in-out;
        }

        .show form h2 {
            font-size: 22px;
        }

        .show form h2 small {
            font-size: 0;
            color: transparent;
        }

        .form-group {
            position: relative;
            margin: 10px 0;
        }

        .form-group::after {
            position: absolute;
            content: '';
            width: 100%;
            height: 2px;
            left: 0;
            bottom: 0;
            background: linear-gradient(45deg, #ffd900, #f00);
        }

        input[type=text],
        input[type=password] {
            width: 100%;
            padding: 8px 0;
            font-size: 16px;
            background-color: transparent;
            border: none;
            outline: none;
        }

        button[type=submit] {
            width: 100%;
            padding: 8px 0;
            border: none;
            background-color: #f00;
            color: #fff;
            font-size: 16px;
            font-weight: 500;
            border-radius: 5px;
            margin-top: 10px;
            cursor: pointer;
        }

        section aside div#login {
            background-color: #ffd900;
            clip-path: ellipse(350px 70px at 15% 8%);
            height: 35px;
        }

        section aside div#login.show {
            background-color: #fff;
            clip-path: ellipse(350px 350px at 10% 50%);
            height: 200px;
        }

        section aside div#reg {
            background-color: #ffd900;
            clip-path: ellipse(350px 60px at 80% 100%);
            height: 35px;
        }

        section aside div#reg.show {
            background-color: #fff;
            clip-path: ellipse(350px 350px at 10% 50%);
            height: 270px;
        }
    </style>
    <script>
        function toggle() {
            let login = document.getElementById('login');
            let reg = document.getElementById('reg');
            login.classList.toggle('show');
            reg.classList.toggle('show');
        }
    </script>
</head>
<body>
    <section>
        <aside>
            <div id="login" class="show">
                <form action="">
                    <h2 onclick="toggle()">Login<br /><small>or</small></h2>
                    <div class="form-group">
                        <input type="text" name="username" placeholder="Username" />
                    </div>
                    <div class="form-group">
                        <input type="password" name="password" placeholder="Password" />
                    </div>
                    <button type="submit">Login</button>
                </form>
            </div>
            <div id="reg">
                <form action="">
                    <h2 onclick="toggle()"><small>or</small><br />Signup</h2>
                    <div class="form-group">
                        <input type="text" name="fullName" placeholder="Full Name" />
                    </div>
                    <div class="form-group">
                        <input type="text" name="username" placeholder="Username" />
                    </div>
                    <div class="form-group">
                        <input type="password" name="password" placeholder="Password" />
                    </div>
                    <button type="submit">Join Now</button>
                </form>
            </div>
        </aside>
    </section>
</body>
</html>
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Result Screenshot(s)

Authentication FormWorking Sample0