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HTML 

              
                <div class="cotn_principal">
<div class="cont_centrar">

  <div class="cont_login">
<div class="cont_info_log_sign_up">
      <div class="col_md_login">
<div class="cont_ba_opcitiy">
        
        <h2>LOGIN</h2>  
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p> 
  <button class="btn_login" onclick="change_to_login()">LOGIN</button>
  </div>
  </div>
<div class="col_md_sign_up">
<div class="cont_ba_opcitiy">
  <h2>SIGN UP</h2>

  
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit.</p>

  <button class="btn_sign_up" onclick="change_to_sign_up()">SIGN UP</button>
</div>
  </div>
       </div>

    
    <div class="cont_back_info">
       <div class="cont_img_back_grey">
       <img src="https://images.unsplash.com/42/U7Fc1sy5SCUDIu4tlJY3_NY_by_PhilippHenzler_philmotion.de.jpg?ixlib=rb-0.3.5&q=50&fm=jpg&crop=entropy&s=7686972873678f32efaf2cd79671673d" alt="" />
       </div>
       
    </div>
<div class="cont_forms" >
    <div class="cont_img_back_">
       <img src="https://images.unsplash.com/42/U7Fc1sy5SCUDIu4tlJY3_NY_by_PhilippHenzler_philmotion.de.jpg?ixlib=rb-0.3.5&q=50&fm=jpg&crop=entropy&s=7686972873678f32efaf2cd79671673d" alt="" />
       </div>
 <div class="cont_form_login">
<a href="#" onclick="hidden_login_and_sign_up()" ><i class="material-icons">&#xE5C4;</i></a>
   <h2>LOGIN</h2>
 <input type="text" placeholder="Email" />
<input type="password" placeholder="Password" />
<button class="btn_login" onclick="change_to_login()">LOGIN</button>
  </div>
  
   <div class="cont_form_sign_up">
<a href="#" onclick="hidden_login_and_sign_up()"><i class="material-icons">&#xE5C4;</i></a>
     <h2>SIGN UP</h2>
<input type="text" placeholder="Email" />
<input type="text" placeholder="User" />
<input type="password" placeholder="Password" />
<input type="password" placeholder="Confirm Password" />
<button class="btn_sign_up" onclick="change_to_sign_up()">SIGN UP</button>

  </div>

    </div>
    
  </div>
 </div>
</div>
!

CSS 

              
                /* ------------------------------------ Click on login and Sign Up to  changue and view the effect
---------------------------------------
*/

* {
  margin: 0px auto;
  padding: 0px;
  text-align: center;
  font-family: "Open Sans", sans-serif;
}

.cotn_principal {
  position: absolute;
  width: 100%;
  display: flex;
  height: 100%;
  /* Permalink - use to edit and share this gradient: http://colorzilla.com/gradient-editor/#cfd8dc+0,607d8b+100,b0bec5+100 */
  background: #cfd8dc; /* Old browsers */
  background: -moz-linear-gradient(
    -45deg,
    #cfd8dc 0%,
    #607d8b 100%,
    #b0bec5 100%
  ); /* FF3.6-15 */
  background: -webkit-linear-gradient(
    -45deg,
    #cfd8dc 0%,
    #607d8b 100%,
    #b0bec5 100%
  ); /* Chrome10-25,Safari5.1-6 */
  background: linear-gradient(
    135deg,
    #cfd8dc 0%,
    #607d8b 100%,
    #b0bec5 100%
  ); /* W3C, IE10+, FF16+, Chrome26+, Opera12+, Safari7+ */
  filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#cfd8dc', endColorstr='#b0bec5',GradientType=1 ); /* IE6-9 fallback on horizontal gradient */
}

.cont_centrar {
  display: flex;
  align-self: center;
  width: 100%;
}

.cont_login {
  position: relative;
  width: 640px;
}

.cont_back_info {
  position: relative;
  float: left;
  width: 640px;
  height: 280px;
  overflow: hidden;
  background-color: #fff;
  box-shadow: 1px 10px 30px -10px rgba(0, 0, 0, 0.5);
}

.cont_forms {
  position: absolute;
  overflow: hidden;
  top: 0px;
  left: 0px;
  width: 320px;
  height: 280px;
  background-color: #eee;
  -webkit-transition: all 0.5s;
  -moz-transition: all 0.5s;
  -ms-transition: all 0.5s;
  -o-transition: all 0.5s;
  transition: all 0.5s;
}

.cont_forms_active_login {
  box-shadow: 1px 10px 30px -10px rgba(0, 0, 0, 0.5);
  height: 420px;
  top: -60px;
  left: 0px;
  -webkit-transition: all 0.5s;
  -moz-transition: all 0.5s;
  -ms-transition: all 0.5s;
  -o-transition: all 0.5s;
  transition: all 0.5s;
}

.cont_forms_active_sign_up {
  box-shadow: 1px 10px 30px -10px rgba(0, 0, 0, 0.5);
  height: 420px;
  top: -60px;
  left: 320px;
  -webkit-transition: all 0.5s;
  -moz-transition: all 0.5s;
  -ms-transition: all 0.5s;
  -o-transition: all 0.5s;
  transition: all 0.5s;
}

.cont_img_back_grey {
  position: absolute;
  width: 950px;
  top: -80px;
  left: -116px;
}

.cont_img_back_grey > img {
  width: 100%;
  -webkit-filter: grayscale(100%);
  filter: grayscale(100%);
  opacity: 0.2;
  animation-name: animar_fondo;
  animation-duration: 20s;
  animation-timing-function: linear;
  animation-iteration-count: infinite;
  animation-direction: alternate;
}

.cont_img_back_ {
  position: absolute;
  width: 950px;
  top: -80px;
  left: -116px;
}

.cont_img_back_ > img {
  width: 100%;
  opacity: 0.3;
  animation-name: animar_fondo;
  animation-duration: 20s;
  animation-timing-function: linear;
  animation-iteration-count: infinite;
  animation-direction: alternate;
}

.cont_forms_active_login > .cont_img_back_ {
  top: -20px;
  -webkit-transition: all 0.5s;
  -moz-transition: all 0.5s;
  -ms-transition: all 0.5s;
  -o-transition: all 0.5s;
  transition: all 0.5s;
}

.cont_forms_active_sign_up > .cont_img_back_ {
  top: -20px;
  left: -435px;
  -webkit-transition: all 0.5s;
  -moz-transition: all 0.5s;
  -ms-transition: all 0.5s;
  -o-transition: all 0.5s;
  transition: all 0.5s;
}

.cont_info_log_sign_up {
  position: absolute;
  width: 640px;
  height: 280px;
  top: 0px;
  z-index: 1;
}

.col_md_login {
  position: relative;
  float: left;
  width: 50%;
}

.col_md_login > h2 {
  font-weight: 400;
  margin-top: 70px;
  color: #757575;
}

.col_md_login > p {
  font-weight: 400;
  margin-top: 15px;
  width: 80%;
  color: #37474f;
}

.btn_login {
  background-color: #26c6da;
  border: none;
  padding: 10px;
  width: 200px;
  border-radius: 3px;
  box-shadow: 1px 5px 20px -5px rgba(0, 0, 0, 0.4);
  color: #fff;
  margin-top: 10px;
  cursor: pointer;
}

.col_md_sign_up {
  position: relative;
  float: left;
  width: 50%;
}

.cont_ba_opcitiy > h2 {
  font-weight: 400;
  color: #fff;
}

.cont_ba_opcitiy > p {
  font-weight: 400;
  margin-top: 15px;
  color: #fff;
}
/* ----------------------------------
background text    
------------------------------------
 */
.cont_ba_opcitiy {
  position: relative;
  background-color: rgba(120, 144, 156, 0.55);
  width: 80%;
  border-radius: 3px;
  margin-top: 60px;
  padding: 15px 0px;
}

.btn_sign_up {
  background-color: #ef5350;
  border: none;
  padding: 10px;
  width: 200px;
  border-radius: 3px;
  box-shadow: 1px 5px 20px -5px rgba(0, 0, 0, 0.4);
  color: #fff;
  margin-top: 10px;
  cursor: pointer;
}
.cont_forms_active_sign_up {
  z-index: 2;
}

@-webkit-keyframes animar_fondo {
  from {
    -webkit-transform: scale(1) translate(0px);
    -moz-transform: scale(1) translate(0px);
    -ms-transform: scale(1) translate(0px);
    -o-transform: scale(1) translate(0px);
    transform: scale(1) translate(0px);
  }
  to {
    -webkit-transform: scale(1.5) translate(50px);
    -moz-transform: scale(1.5) translate(50px);
    -ms-transform: scale(1.5) translate(50px);
    -o-transform: scale(1.5) translate(50px);
    transform: scale(1.5) translate(50px);
  }
}
@-o-keyframes identifier {
  from {
    -webkit-transform: scale(1);
    -moz-transform: scale(1);
    -ms-transform: scale(1);
    -o-transform: scale(1);
    transform: scale(1);
  }
  to {
    -webkit-transform: scale(1.5);
    -moz-transform: scale(1.5);
    -ms-transform: scale(1.5);
    -o-transform: scale(1.5);
    transform: scale(1.5);
  }
}
@-moz-keyframes identifier {
  from {
    -webkit-transform: scale(1);
    -moz-transform: scale(1);
    -ms-transform: scale(1);
    -o-transform: scale(1);
    transform: scale(1);
  }
  to {
    -webkit-transform: scale(1.5);
    -moz-transform: scale(1.5);
    -ms-transform: scale(1.5);
    -o-transform: scale(1.5);
    transform: scale(1.5);
  }
}
@keyframes identifier {
  from {
    -webkit-transform: scale(1);
    -moz-transform: scale(1);
    -ms-transform: scale(1);
    -o-transform: scale(1);
    transform: scale(1);
  }
  to {
    -webkit-transform: scale(1.5);
    -moz-transform: scale(1.5);
    -ms-transform: scale(1.5);
    -o-transform: scale(1.5);
    transform: scale(1.5);
  }
}
.cont_form_login {
  position: absolute;
  opacity: 0;
  display: none;
  width: 320px;
  -webkit-transition: all 0.5s;
  -moz-transition: all 0.5s;
  -ms-transition: all 0.5s;
  -o-transition: all 0.5s;
  transition: all 0.5s;
}

.cont_forms_active_login {
  z-index: 2;
}

.cont_form_sign_up {
  position: absolute;
  width: 320px;
  float: left;
  opacity: 0;
  display: none;
  -webkit-transition: all 0.5s;
  -moz-transition: all 0.5s;
  -ms-transition: all 0.5s;
  -o-transition: all 0.5s;
  transition: all 0.5s;
}

.cont_form_sign_up > input {
  text-align: left;
  padding: 15px 5px;
  margin-left: 10px;
  margin-top: 20px;
  width: 260px;
  border: none;
  color: #757575;
}

.cont_form_sign_up > h2 {
  margin-top: 50px;
  font-weight: 400;
  color: #757575;
}

.cont_form_login > input {
  padding: 15px 5px;
  margin-left: 10px;
  margin-top: 20px;
  width: 260px;
  border: none;
  text-align: left;
  color: #757575;
}

.cont_form_login > h2 {
  margin-top: 110px;
  font-weight: 400;
  color: #757575;
}
.cont_form_login > a,
.cont_form_sign_up > a {
  color: #757575;
  position: relative;
  float: left;
  margin: 10px;
  margin-left: 30px;
}
!

JS 

              
                /* ------------------------------------ Click on login and Sign Up to  changue and view the effect
---------------------------------------
*/

const time_to_show_login = 400;
const time_to_hidden_login = 200;

function change_to_login() {
document.querySelector('.cont_forms').className = "cont_forms cont_forms_active_login";  
document.querySelector('.cont_form_login').style.display = "block";
document.querySelector('.cont_form_sign_up').style.opacity = "0";               

setTimeout(function(){  document.querySelector('.cont_form_login').style.opacity = "1"; },time_to_show_login);  
  
setTimeout(function(){    
document.querySelector('.cont_form_sign_up').style.display = "none";
},time_to_hidden_login);  
  }

  const time_to_show_sign_up = 100;
  const time_to_hidden_sign_up = 400;

function change_to_sign_up(at) {
  document.querySelector('.cont_forms').className = "cont_forms cont_forms_active_sign_up";
  document.querySelector('.cont_form_sign_up').style.display = "block";
document.querySelector('.cont_form_login').style.opacity = "0";
  
setTimeout(function(){  document.querySelector('.cont_form_sign_up').style.opacity = "1";
},time_to_show_sign_up);  

setTimeout(function(){   document.querySelector('.cont_form_login').style.display = "none";
},time_to_hidden_sign_up);  


}    

const time_to_hidden_all = 500;

function hidden_login_and_sign_up() {

document.querySelector('.cont_forms').className = "cont_forms";  
document.querySelector('.cont_form_sign_up').style.opacity = "0";               
document.querySelector('.cont_form_login').style.opacity = "0"; 

setTimeout(function(){
document.querySelector('.cont_form_sign_up').style.display = "none";
document.querySelector('.cont_form_login').style.display = "none";
},time_to_hidden_all);  
  
  }
!
999px

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