Мне кажется если переделать этот код то будет выглядеть очень круто.  
Код
<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="cambiar_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="cambiar_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="ocultar_login_sign_up()" ><i class="material-icons"></i></a> 
  <h2>LOGIN</h2> 
  <input type="text" placeholder="Email" /> 
 <input type="password" placeholder="Password" /> 
 <button class="btn_login" onclick="cambiar_login()">LOGIN</button> 
  </div> 
   
  <div class="cont_form_sign_up"> 
 <a href="#" onclick="ocultar_login_sign_up()"><i class="material-icons"></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="cambiar_sign_up()">SIGN UP</button> 
  </div> 
  </div> 
   
  </div> 
  </div> 
 </div>
   Код
/* ------------------------------------ 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%; 
  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 { 
  position: relative; 
  float: left; 
  width: 100%; 
 } 
 .cont_login { 
  position: relative; 
  width: 640px; 
 left: 50%; 
 margin-left: -320px; 
   
 } 
 .cont_back_info {  
 position: relative; 
  float: left; 
  width: 640px; 
  height: 280px; 
 overflow: hidden; 
  background-color: #fff; 
  margin-top: 100px; 
 box-shadow: 1px 10px 30px -10px rgba(0,0,0,0.5); 
 } 
 .cont_forms { 
  position: absolute; 
  overflow: hidden; 
  top:100px; 
 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:20px; 
 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:20px; 
 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: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 > .cont_img_back_ { 
 top:0px;  
 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: 100px; 
 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_forms_active_login >.cont_form_login { 
 } 
 .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; 
 }
   Код
/* ------------------------------------ Click on login and Sign Up to changue and view the effect 
 --------------------------------------- 
 */ 
 function cambiar_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"; },400);  
   
 setTimeout(function(){  
 document.querySelector('.cont_form_sign_up').style.display = "none"; 
 },200);  
  } 
 function cambiar_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"; 
 },100);  
 setTimeout(function(){ document.querySelector('.cont_form_login').style.display = "none"; 
 },400);  
 }  
 function ocultar_login_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"; 
 },500);  
   
  }