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HTML 

              
                <div class="main-wrapper">
  
    <div class="card-container">
        <div class="card">
            <div class="login-form">
                <div class="header">Log in</div>
                <div class="content">
                    <form>
                        <div class="input-field" >
                            <input type="text" placeholder="Username">
                        </div>
                        <div class="input-field">
                            <input type="password" placeholder="Password">
                        </div>
                        <div class="input-field">
                            <button class="btn btn-submit" type="submit">Log in</button>
                        </div>
                    </form>
                </div>
                <div class="footer">
                    Don't have an account ?
                    <button class="btn btn-rotate" id="btn-signup">Sign up</button>
                </div>
            </div> <!-- end login-form panel -->
            <div class="signup-form">
                <div class="header">Sign up</div>
                <div class="content">
                    <form>
                        <div class="input-field" >
                            <input type="text" placeholder="Username">
                        </div>
                        <div class="input-field" >
                            <input type="email" placeholder="Email">
                        </div>
                        <div class="input-field group">
                            <input type="password" placeholder="Password">
                            <span class="see-password">
                                <svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24">
                                    <path d="M0 0h24v24H0z" fill="none"/>
                                    <path fill="#fff" d="M12 4.5C7 4.5 2.73 7.61 1 12c1.73 4.39 6 7.5 11 7.5s9.27-3.11 11-7.5c-1.73-4.39-6-7.5-11-7.5zM12 17c-2.76 0-5-2.24-5-5s2.24-5 5-5 5 2.24 5 5-2.24 5-5 5zm0-8c-1.66 0-3 1.34-3 3s1.34 3 3 3 3-1.34 3-3-1.34-3-3-3z"/>
                                </svg>
                            </span>
                        </div>
                        <div class="input-field">
                            <button class="btn btn-submit" type="submit">Get started</button>
                        </div>
                    </form>
                </div>
                <div class="footer">
                    <button class="btn btn-rotate" id="btn-login">I have an account</button>
                </div>
            </div> <!-- end signup-form panel -->
        </div> <!-- end card -->
    </div> <!-- end card-container -->
</div>
!

CSS 

              
                .main-wrapper {
  width: 100vw;
  overflow: hidden;
  min-height: 100vh;
  display: flex;
  justify-conent: center;
  align-items: center;
	font-size: 14px;
	font-family: "Helvetica Nueue",Arial,Verdana,sans-serif;
    /*background:  url(https://github.com/amaury-diallo/pure-javascript-todolist-app/blob/master/blue-bg.svg) no-repeat center; */
  background: #9D50BB;  /* fallback for old browsers */
background: -webkit-linear-gradient(to right, #6E48AA, #9D50BB);  /* Chrome 10-25, Safari 5.1-6 */
background: linear-gradient(to right, #6E48AA, #9D50BB); /* W3C, IE 10+/ Edge, Firefox 16+, Chrome 26+, Opera 12+, Safari 7+ */
 z-index: 0;
}

/* entire container, keeps perspective */
.card-container {
    perspective: 500px;
    margin-bottom: 30px;
}
/* flip the pane when hovered */
.card-container.rotate .card{
    transform: rotateY( 180deg );
}
/* flip speed goes here */
.card {
    transition: transform .85s;
    transform-style: preserve-3d;
	position: relative;
}

/* hide back of pane during swap */
.login-form, .signup-form {
    backface-visibility: hidden;
	position: absolute;
	top: 0;
	left: 0;
	background-color: #FFF;
    box-shadow: 0 1px 3px 2px rgba(0, 0, 0, 0.2);
    display: flex;
    flex-direction: column;
}

/* front pane, placed above back */
.login-form {
	z-index: 2;
}

/* back, initially hidden pane */
.signup-form {
    transform: rotateY(180deg);
    z-index: 3;
}

/*        Style       */
.card-container{
    width: 300px;
    margin: 0 auto;
}

.card{
    background: #FFFFFF;
    border-radius: 4px;
    color: #444444;
}

.card-container, .login-form, .signup-form {
	height: 420px;
	border-radius: 4px;
}
.login-form, .signup-form {
    width: 100%;
    height: 420px;
    border-radius: 4px;
}
.card .header {
    margin: 10px;
    padding: 10px 0 10px 0;
    text-align: center;
    border-bottom: 1px solid #EEEEEE;
    color: #999999;
    text-transform: uppercase;
    font-weight: bold;
    box-shadow: 0 16px 24px 2px rgba(0, 0, 0, 0.14), 0 6px 30px 5px rgba(0, 0, 0, 0.12), 0 8px 10px -7px rgba(0, 0, 0, 0.2);
}
.card .content{
    display: flex;
    align-items: center;
    justify-content: center;
    background-color: rgba(0, 0, 0, 0);
    box-shadow: none;
    flex: 1;
}
.card .footer {
    border-top: 1px solid #EEEEEE;
    color: #999999;
    padding: 5px 0 5px 0;
    text-align: center;
}

.btn {
    border: none;
    border-radius: 2px;
    display: inline-block;
    height: 36px;
    line-height: 36px;
    padding: 0 16px;
    font-family: inherit;
    font-weight: 100;
    text-align: center;
    vertical-align: middle;
    user-select: none;
    outline: none;
    border: none;
    cursor: pointer;
    transition: .4s;
}

.btn-rotate {
    color: #212529;
    background-color: #ddd;
}

.btn-rotate:hover {
    color: #212529;
    background-color: #ccc;
}

.btn-rotate:focus{
    outline: none;
    border: none;
}

.btn-submit {
    width: 50%;
    margin: 0 auto;
    border-radius: 20px;
    text-transform: uppercase;
    color: #212529;
    font-weight: bold;
    background-color: #809BE0;
}

.btn-submit:hover {
    color: #212529;
    background-color: #4D8CDE;
}

.btn-submit:focus{
    outline: none;
    border: none;
}

form{
    width: 100%;
}

.input-field{
    position: relative;
    width: 90%;
    margin: 20px auto;
    padding: 0;
    display: flex;
    align-items: center;
    justify-content: center;
}

.input-field input{ 
    width: 100%;
    max-height: 20px;
    padding: 20px;
    margin-bottom: 10px;
    border: none;
    outline: none;
    border-radius: 30px;
    background-color: #A2B2DE;
    color: #fff;
    transition: .4s;
}

::-webkit-input-placeholder {
  color: #fff;
}
::-moz-placeholder {
  color: #fff;
}
:-ms-input-placeholder {
  color: #fff;
}
::-ms-input-placeholder {
  color: #fff;
}
::placeholder {
  color: #fff;
}

.input-field input:focus{
    box-shadow: 0 2px 2px 0 rgba(0, 0, 0, 0.14), 0 3px 1px -2px rgba(0, 0, 0, 0.12), 0 1px 5px 0 rgba(0, 0, 0, 0.2);
}

.see-password{
    position: absolute;
    right: 10px;
    padding-bottom: 5px;
    cursor: pointer;
}

.group input{
    padding-right: 40px;
}
!

JS 

              
                document.addEventListener('DOMContentLoaded',  (event) => {

  const rotateCard = () => {
    const cardContainer = document.querySelector('.card-container') 
    cardContainer.classList.toggle('rotate')
  }

  const btnSignup = document.querySelector('#btn-signup') ,
        btnLogin = document.querySelector('#btn-login')

  btnSignup.addEventListener('click', rotateCard)
  btnLogin.addEventListener('click', rotateCard)

  /*See passwod*/
  const seePassword =  () => {
    const seePwdIcon = document.querySelector('.see-password'),
          pwdInput = document.querySelector('.group input')

    seePwdIcon.addEventListener('mousedown', () => {
      pwdInput.type = 'text'
    })

    seePwdIcon.addEventListener('mouseup', () => {
      pwdInput.type = 'password'
    })

    seePwdIcon.addEventListener('mouseover', () => {
      pwdInput.type = 'password'
    })
  }

  seePassword()
})
!
999px

Console