<article class="container">
  <!-- Внутренний Блок -->
  <div class="block">
    <section class="block_item block-item">
      <h2 class="block-item_title">У вас уже есть аккаунт?</h2>
      <button class="block-item_btn signin-btn">Войти</button>
    </section>
    <section class="block_item block-item">
      <h2 class="block-item_title">У вас нет аккаунта?</h2>
      <button class="block-item_btn signup-btn">Зарегистрироваться</button>
    </section>
  </div>
  <!-- Блок Формы-->
  <div class="form-box">

  </div>
</article>

* {
  box-sizing: border-box;
}

body {
  font-family: sans-serif;
  margin: 0;

  min-height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;

  background-image: url(кб1.jpg);
  background-size: 205vh;
}
/*Контейнер*/

.container {
  width: 740px;
  height: 50vh;
  min-height: 150px;
  padding: 20px 0;
  position: relative;
}
.block {
  height: 100%;
  background-color: #fff;
  opacity: 1;
  box-shadow: 0 5px 15px rgba(0, 0, 0, 0.2);
  border-radius: 12px;

  display: flex;
  align-items: center;
}
.block_item {
  text-align: center;
  width: 50%;
}
.block-item_title {
  font-size: 1.2rem;
  font-weight: 500;
  color: #000;
  margin-bottom: 10px;
}
.block-item_btn {
  border: none;
  cursor: pointer;
  padding: 10px 20px;
  background-color: #f2f3f4;
  color: #000;
  font-size: 16px;
  font-weight: 500;
  -webkit-border-radius: 20px;
  -moz-border-radius: 20px;
  border-radius: 20px;
}
/*Форма*/
.form-box {
  background-color: #fff;
  height: 100%;
  width: 50%;
  box-shadow: 0 5px 15px rgba(0,0,0,0.15);

  position: absolute;
  top: 0;
  left: 0;
  z-index: 1000;
}
.active {
  left: 50%;
}

const signinbtn = document.querySelector('.signin-btn');
const signupbtn = document.querySelector('.signup-btn');
const formbox = document.querySelector('.form-box');

signupbtn.addEventListener("click", function () {
  formbox.classList.add("active");
});

signinbtn.addEventListener("click", function () {
  formbox.classList.remove("active");
});