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CSS

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HTML 

              
                <div class="main-container">
      <div id="sign-up" class="left-container sign-up hidden">
        <h1>So let's sign up</h1>
        <div style="width: 100%;">
          <form action="index.html" class="signup-form">
            <input type="text" id="first-name" placeholder="First Name">
            <input type="text" id="last-name" placeholder="Last Name">
            <input type="email" id="email" placeholder="E-mail">
            <input type="password" id="password" placeholder="Password">
            <div class="check">
              <input type="checkbox" onclick="checkTerms()" id="agree-terms">
              <label for="agree-terms">Agree to
                <a href="#">Terms &amp; Conditions</a>
              </label>
            </div>
            <button type="submit" id="signup_submit" class="button sign-up-btn disabled" disabled>Sign Up</button>
          </form>
        </div>
        <div class="signin-link-container">
          <p  onclick="changeToLogin()" class="signin-link">ALready have an account? Click here!</p>
        </div>
      </div>

      <div id="log-in" class="left-container log-in">
        <h1>So let's log in</h1>
        <div style="width: 100%;">
          <form action="index.html" class="signup-form">
            <input type="email" id="email" placeholder="E-mail">
            <input type="password" id="password" placeholder="Password">
            <button type="submit" id="submit" class="button log-in-btn" disabled>Login</button>
          </form>
        </div>
        <div class="login-link-container">
          <p onclick="changeToSignup()" class="login-link">Don't have an account? Click here!</p>
        </div>
      </div>

      <div class="right-container"></div>
    </div>
!

CSS 

              
                body {
  background-image: linear-gradient(to top right, #3f51b5, #e91e63);
  min-height: 100vh;
  margin: 0;
  font-family: 'Secular One', sans-serif;
}
h1{
  margin: 0;
}
a {
  text-decoration: none;
  color: #0d28c0;
}

a:hover{
  color: #071561;
}

.main-container {
  display: flex;
  width: 70%;
  height: 700px;
  min-height: 80vh;
  background-color: white;
  margin: 10vh auto;
  border-radius: 12px;
  box-shadow: 5px 5px 20px 0px rgba(0, 0, 0, 0.5);
}

.main-container .hidden{
  display: none;
}

.login-link {
  color: #777;
}

.login-link:hover {
  color: #333;
  cursor: pointer;
}

.left-container {
  width: 40%;
  height: 100%;
  background-color: white;
  border-radius: 10px 0 0 10px;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: space-around;
}

left-container > * {
  transition: all 0.3s;
}

.right-container {
  width: 60%;
  height: 100%;
  background-image: url('https://images.unsplash.com/photo-1519408469771-2586093c3f14?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=2147&q=80');
  background-position: center;
  background-size: cover;
  border-radius: 0 10px 10px 0;
}
.signup-form {
  width: 70%;
  margin: auto;
  display: flex;
  flex-direction: column;
  align-items: center;
}

.signup-form input {
  border-radius: 200px;
  padding: 5px 15px;
  height: 2rem;
  font-size: 1rem;
  background-color: #ddd;
  color: #444;
  border: none;
}

.signup-form input {
  display: block;
  margin-top: 1rem;
  width: 100%;
  font-family: inherit;
}

.signup-form input.first-name,
.signup-form input.last-name {
  width: 48%;
}

.signup-form input:focus {
  outline: none;
}

.signup-form input[type='checkbox']{
  margin: 1rem 0.5rem 1rem 0;
}

.signup-form input[type='checkbox'],
.signup-form input[type='checkbox'] + label {
  display: inline-block;
  width: auto;
}

.signup-form .check{
  display: flex;
  align-items: center;
}

.signup-form button {
  color: white;
  background-color: #bf360c;
  border: none;
  padding: 1rem;
  font-family: inherit;
  border-radius: 10px;
  font-size: 1.5rem;
  display: block;
  width: 50%;
  margin: auto;
  margin-top: 2rem;
}

.signup-form button:hover{
  background-color: #83260a;
  cursor: pointer;
}

button.disabled {
  color: grey;
  background-color: darkgrey;
}

button.disabled:hover {
  color: grey;
  background-color: darkgrey;
  cursor: unset;
}

.signin-link {
  color: #777;
}

.signin-link:hover {
  color: #333;
  cursor: pointer;
}

@media (max-width:1240px){
  .left-container{
    width: 50%;
  }
  .right-container{
    width: 50%;
  }
}

@media (max-width:970px){
  .signup-form button{
    font-size: 1rem;
  }
  .signup-form input[type="checkbox"] + label{
    font-size: 0.8rem;
  }
  .signin-link{
    font-size: 0.8rem;
  }
}

@media (max-width:950px) {
  .left-container{
    width: 100%;
    border-radius: 10px;
  }
  .main-container h1{
    font-size: 1.5rem;
    margin-top: 0.5rem;
  }
  .signup-form input{
    margin-top: 0.5rem;
  }
  .right-container{
    display: none;
  }
}

@media (max-width:550px){
  .main-container{
    width: 90%;
    left: 5%;
  }
}
!

JS 

              
                const checkTerms = () => {
  let termsCheck = document.getElementById('agree-terms');
  let btn = document.getElementById('signup_submit');
  if (termsCheck.checked == true){
    btn.classList.remove("disabled");
    btn.disabled = false;
  }else{
    btn.classList.add("disabled");
    btn.disabled = true;
  }
}

let signUp = document.getElementById("sign-up");
let logIn = document.getElementById("log-in");

const changeToLogin = () => {
  signUp.classList.add("hidden");
  logIn.classList.remove("hidden");
}

const changeToSignup = () => {
  signUp.classList.remove("hidden");
  logIn.classList.add("hidden");
}
!
999px

Console