index.html

<section>
  <div class="background"></div>
  <div class="box">
    <div class="square" style="--i: 0"></div>
    <div class="square" style="--i: 1"></div>
    <div class="square" style="--i: 2"></div>
    <div class="square" style="--i: 3"></div>
    <div class="square" style="--i: 4"></div>
    <div class="container">
      <div class="form">
        <h2>Login Form</h2>
        <form>
          <div class="input__box">
            <input type="text" placeholder="Username" />
          </div>
          <div class="input__box">
            <input type="password" placeholder="Password" />
          </div>
          <div class="input__box">
            <input type="submit" value="Login" />
          </div>
          <p class="forget">
            Forgot Password? <a href="#">Click Here</a>
          </p>
          <p class="forget">
            Don't have an account? <a href="#">Sign Up</a>
          </p>
        </form>
      </div>
    </div>
  </div>
</section>

<style>
  * {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: "Poppins", sans-serif;
  }

  body {
    overflow: hidden;
  }

  section {
    display: flex;
    min-height: 100vh;
    align-items: center;
    justify-content: center;
    background: linear-gradient(to bottom, #17263c, #0f1b2b);
  }

  section .background {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: -1;
    opacity: 0.8;
    background: linear-gradient(135deg, #e94057, #ffa500, #18a0fb);
    animation: gradientAnimation 20s ease infinite;
  }

  @keyframes gradientAnimation {
    0% {
      background-position: 0% 50%;
    }
    50% {
      background-position: 100% 50%;
    }
    100% {
      background-position: 0% 50%;
    }
  }

  .box {
    position: relative;
  }

  .box .square {
    position: absolute;
    border-radius: 10px;
    backdrop-filter: blur(5px);
    background: rgba(255, 255, 255, 0.1);
    animation-delay: calc(-1s * var(--i));
    animation: animate 8s linear infinite alternate;
    box-shadow: 0 25px 45px rgba(0, 0, 0, 0.2);
    border: 1px solid rgba(255, 255, 255, 0.3);
  }

  @keyframes animate {
    0%,
    100% {
      transform: scale(1);
    }
    50% {
      transform: scale(1.1);
    }
  }

  .box .square:nth-child(1) {
    top: -50px;
    left: -60px;
    width: 100px;
    height: 100px;
    background: #e94057;
  }

  .box .square:nth-child(2) {
    z-index: 2;
    top: 150px;
    left: -100px;
    width: 120px;
    height: 120px;
    background: #ffa500;
  }

  .box .square:nth-child(3) {
    z-index: 2;
    width: 80px;
    height: 80px;
    right: -50px;
    bottom: -60px;
    background: #18a0fb;
  }

  .box .square:nth-child(4) {
    left: 100px;
    width: 50px;
    height: 50px;
    bottom: -80px;
    background: #e94057;
  }

  .box .square:nth-child(5) {
    top: -80px;
    left: 140px;
    width: 60px;
    height: 60px;
    background: #ffa500;
  }

  .container {
    width: 400px;
    display: flex;
    min-height: 400px;
    position: relative;
    border-radius: 10px;
    align-items: center;
    justify-content: center;
    backdrop-filter: blur(5px);
    background: rgba(255, 255, 255, 0.2);
    box-shadow: 0 25px 45px rgba(0, 0, 0, 0.2);
    border: 1px solid rgba(255, 255, 255, 0.3);
  }

  .form {
    width: 100%;
    height: 100%;
    padding: 40px;
    position: relative;
  }

  .form h2 {
    color: #fff;
    font-size: 24px;
    font-weight: 600;
    position: relative;
    letter-spacing: 1px;
    margin-bottom: 40px;
  }

  .form h2::before {
    left: 0;
    width: 80px;
    height: 4px;
    content: "";
    bottom: -10px;
    background: #fff;
    position: absolute;
  }

  .form .input__box {
    width: 100%;
    margin-top: 20px;
  }

  .form .input__box input {
    width: 100%;
    color: #fff;
    border: none;
    outline: none;
    font-size: 16px;
    padding: 10px 20px;
    letter-spacing: 1px;
    border-radius: 35px;
    background: rgba(255, 255, 255, 0.2);
    border: 1px solid rgba(255, 255, 255, 0.3);
    box-shadow: 0 5px 15px rgba(0, 0, 0, 0.2);
  }

  .form .input__box input[type="submit"] {
    color: #666;
    cursor: pointer;
    background: #fff;
    max-width: 100px;
    font-weight: 600;
    margin-bottom: 20px;
  }

  .forget {
    color: #fff;
    margin-top: 5px;
  }

  .forget a {
    color: #fff;
    font-weight: 600;
    text-decoration: none;
  }
</style>