<form action="javascript:void(0);" class="login-form">
  <h1>Login</h1>
  <div class="form-input-material">
    <input type="text" name="username" id="username" placeholder=" " autocomplete="off" required class="form-control-material" />
    <label for="username">Username</label>
  </div>
  <div class="form-input-material">
    <input type="password" name="password" id="password" placeholder=" " autocomplete="off" required class="form-control-material" />
    <label for="password">Password</label>
  </div>
  <button type="submit" class="btn btn-ghost">Login</button>
</form>

* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
}

:root {
  --primary-color: hsl(196, 78%, 61%);
}

body {
  min-height: 100vh;
  background: #eceffc;
  display: flex;
  justify-content: center;
  align-items: center;
}

.login-form {
  display: flex;
  flex-direction: column;
  align-items: center;
  padding: 50px 40px;
  color: white;
  font-family: "Microsoft YaHei", serif;
  font-size: 16px;
  background-color: rgba(0, 0, 0, 0.8);
  border-radius: 8px;
  box-shadow: 0 0.4px 0.4px rgba(128, 128, 128, 0.109),
    0 1px 1px rgba(128, 128, 128, 0.155),
    0 2.1px 2.1px rgba(128, 128, 128, 0.195),
    0 4.4px 4.4px rgba(128, 128, 128, 0.241),
    0 12px 12px rgba(128, 128, 128, 0.35);

  h1 {
    margin: 0 0 24px 0;
  }

  .form-input-material {
    margin: 12px 0;
  }

  .btn {
    width: 100%;
    margin: 18px 0 9px 0;
    padding: 8px 20px;
    position: relative;
    border-radius: 0;
  }

  input {
    color: white;
  }

  button,
  input {
    font: inherit;
    outline: none;
  }
}

.form-input-material {
  --input-border-bottom-color: white;

  position: relative;
  border-bottom: 1px solid var(--input-border-bottom-color);

  &::before {
    position: absolute;
    content: "";
    left: 0;
    bottom: -1px;
    width: 100%;
    height: 2px;
    background-color: var(--primary-color);
    transform: scaleX(0);
    transform-origin: center;
    transition: 0.5s;
  }

  &:focus-within::before {
    transform: scaleX(1);
  }

  .form-control-material {
    padding: 0.5rem 0;
    background: none;
    border: none;

    // input 处于focus状态 or 占位符未显示(即input中输入了数据)
    &:focus,
    &:not(:placeholder-shown) {
      ~ label {
        transform: translateY(-110%) scale(0.8);
        color: var(--primary-color);
      }
    }
  }

  label {
    position: absolute;
    top: 0.5rem;
    left: 0;
    transition: 0.3s;
    transform-origin: left;
  }
}

.btn-ghost {
  --btn-color: var(--primary-color);
  --btn-border-color: var(--primary-color);

  background: none;
  transition: 0.3s;
  overflow: hidden;   // important!
  color: var(--btn-color);
  border: 1px solid var(--btn-border-color);

  &::before {
    position: absolute;
    content: "";
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: linear-gradient(
      120deg,
      transparent,
      var(--primary-color),
      transparent
    );
    transform: translateX(-100%);
    transition: 0.6s;
  }

  &:hover {
    box-shadow: 0 0 20px 5px hsla(204, 70%, 53%, 0.5);

    &::before {
      // translateX(-100%) --> translateX(100%)
      transform: translateX(100%);
    }
  }
}