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HTML 

              
                <header>
	<h1>a11y sign in&#8725;up</h1>
	<p class="page-title--sub">(desktop only)</p>
</header>
<main class="container" id="container">
	<!--   Sign in -->
	<section class="form-container sign-in-container">
		<form id="signInForm">
			<fieldset>
				<legend>
					<h1>Sign in</h1>
				</legend>
				<section role="group" aria-label="Sign in using a social media account" class="social-container">
					<a id="socialSignIn" href="#" class="social" aria-label="Facebook account" title="Facebook sign in">
						<i class="fab fa-facebook-f"></i></a>
					<a href="#" class="social" aria-label="Google+ account" title="Google+ sign in">
						<i class="fab fa-google-plus-g"></i></a>
					<a href="#" class="social" aria-label="Linkedin account" title="Linkedin sign in">
						<i class="fab fa-linkedin-in"></i></a>
				</section>
				<section role="group" aria-labelledby="signInEmailFields">
					<h2 id="signInEmailFields" class="sub-section__title">or use your email<span class="a11ytxt"> to sign in</span></h2>
					<label for="signInEmail">email</label>
					<input id="signInEmail" name="email" type="email" autocomplete="email" />
					<label for="signInPassword">password</label>
					<input id="signInPassword" name="password" type="password" autocomplete="current-password" />
					<p class="password-reminder">
						<a href="#">Forgot your password?</a>
					</p>
				</section>
			</fieldset>
			<button type="submit">Sign In</button>
		</form>
	</section>
	<!--   Sign up -->
	<section class="form-container sign-up-container">
		<form id="signUpForm">
			<fieldset>
				<legend>
					<h1>Create an account</h1>
				</legend>
				<section role="group" aria-label="Sign up using a social media account" class="social-container">
					<a id="socialSignUp" href="#" class="social" aria-label="Facebook account">
						<i class="fab fa-facebook-f"></i></a>
					<a href="#" class="social" aria-label="Google+ account">
						<i class="fab fa-google-plus-g"></i></a>
					<a href="#" class="social" aria-label="Linkedin account">
						<i class="fab fa-linkedin-in"></i></a>
				</section>
				<section role="group" aria-labelledby="signUpEmailFields">
				<h2 id="signUpEmailFields" class="sub-section__title">or use your email <span class="a11ytxt">to sign up</span></h2>
				<label for="username">username</label>
				<input id="username" type="text" name="username" autocomplete="username" />
				<label for="signUpEmail">email</label>
				<input id="signUpEmail" type="email" name="email" autocomplete="email" />
				<label for="signUpPassword">password</label>
				<input id="signUpPassword" type="password" name="password" autocomplete="new-password" />
					</section>
				<button type="submit">Sign Up</button>
			</fieldset>
		</form>
	</section>
	<div class="overlay-container">
		<div class="overlay">
			<div class="overlay-panel overlay-right">
				<h1 id="signUpOverlayHeading">Don't have an account?</h1>
				<button class="ghost" id="signUp" aria-describedby="signUpOverlayHeading">Sign Up</button>
			</div>
			<div class="overlay-panel overlay-left">
				<h1 id="signInOverlayHeading">Already have an account?</h1>
				<button class="ghost" id="signIn" tabindex="-1" aria-describedby="signInOverlayHeading">Sign In</button>
			</div>
		</div>
	</div>
</main>

<footer>
	<p>Made accessible 👩‍💻 by <a href="https://github.com/krisztin" target="_blank" noreferrer noopener>krisztin</a>, original version by
		<a target="_blank" noreferrer noopener href="https://www.florin-pop.com/blog/2019/03/double-slider-sign-in-up-form/">Florin Pop</a>. <br> Read about <a href="https://www.kriszt.in/blog/a11yform" target="_blank" noreferrer noopener>how I've done it and why</a>.
	</p>
</footer>
!

CSS 

              
                @import url('https://fonts.googleapis.com/css?family=Montserrat:400,800');

:root {
  --colour-primary: #E00034;
	/*had to tweak primary colour as it had barely any contrast*/
  --colour-grey: #2b2b2b;
  --colour-grey--light: #a8a8a8;
	--colour-focus: #ffdd00;
	
	--font: 'Montserrat', sans-serif;
}

* {
	-webkit-box-sizing: border-box;
	        box-sizing: border-box;
}

body {
	background: #f6f5f7;
	display: -webkit-box;
	display: -ms-flexbox;
	display: flex;
	-webkit-box-pack: center;
	    -ms-flex-pack: center;
	        justify-content: center;
	-webkit-box-align: center;
	    -ms-flex-align: center;
	        align-items: center;
	-webkit-box-orient: vertical;
	-webkit-box-direction: normal;
	    -ms-flex-direction: column;
	        flex-direction: column;
	font-family: var(--font);
	height: 100%;
	margin: 0 0 10rem;
}

fieldset {
	border: none;
	padding: 0;
	width: 100%;
}

section {
  width: 100%;
}

h1 {
	margin: 0;
	padding-top: 1em;
  font-size: 2rem;
}

header {
  max-width: 500px;
  text-align: center;
}

.page-title--sub {
	margin: 0 0 2rem;
}

.a11ytxt {
  border: 0;
  clip: rect(0 0 0 0);
  height: 1px;
  margin: -1px;
  overflow: hidden;
  padding: 0;
  position: absolute;
  width: 1px;
}

.sub-section__title {
  font-size: 0.9rem;
  font-weight: normal;
  position: relative;
  display: inline-block;
  margin: 0.5rem 0;
}


/* Horizontal line before and after the section 
divider "or use your email"*/
.sub-section__title:after,
.sub-section__title:before{
  position: absolute;
  content: '';
  width: 40%;
  height: 1px;
  background: var(--colour-grey--light);
  top: 0.45rem;
}
.sub-section__title:after {  
  margin-left: 1rem;
  left: 100%;
}
.sub-section__title:before {
  margin-right: 1rem;
  right: 100%;
}

/*****
 anchor links
*****/

a {
  color: var(--colour-primary);
  font-weight: bold;
}

a:focus {
	background: var(--colour-focus);
	color: #000;
}

/* Social links */
a.social {
  font-size: 1.2rem;
	color: #000;
	text-decoration: none;
	margin: 1rem 0;
}

a.social:hover {
  color: var(--colour-primary);
  border: 3px solid var(--colour-primary);
}

a.social:active {
	color: #000;
  background-color: var(--colour-primary);
}

.social-container {
	margin: 1.5rem 0 0.5rem;
}

.social-container a {
	border: 1px solid var(--colour-grey--light);
	border-radius: 50%;
	display: -webkit-inline-box;
	display: -ms-inline-flexbox;
	display: inline-flex;
	-webkit-box-pack: center;
	    -ms-flex-pack: center;
	        justify-content: center;
	-webkit-box-align: center;
	    -ms-flex-align: center;
	        align-items: center;
	margin: 0 0.5rem;
	height: 3rem;
	width: 3rem;
}

.password-reminder {
  text-align: left;
  margin: 0 0 1rem;
}

.password-reminder a {
  font-size: 0.9rem;
}

/*****
 buttons
*****/

button {
  cursor: pointer;
	border-radius: 20px;
	border: 1px solid var(--colour-primary);
	background-color: var(--colour-primary);
	color: #FFFFFF;
  font-family: inherit;
  font-size: 0.85rem;
	font-weight: bold;
	padding: 12px 45px;
	letter-spacing: 1px;
	text-transform: uppercase;
	-webkit-transition: -webkit-transform 80ms ease-in;
	transition: -webkit-transform 80ms ease-in;
	-o-transition: transform 80ms ease-in;
	transition: transform 80ms ease-in;
	transition: transform 80ms ease-in, -webkit-transform 80ms ease-in;
  margin-top: 0.5rem;
}

button:hover {
	-webkit-transform: scale(1.05);
	    -ms-transform: scale(1.05);
	        transform: scale(1.05);
  -webkit-box-shadow: 0 4px 8px rgba(0,0,0,0.15), 
			0 5px 5px rgba(0,0,0,0.12);
          box-shadow: 0 4px 8px rgba(0,0,0,0.15), 
			0 5px 5px rgba(0,0,0,0.12);
}

button:active {
	-webkit-transform: scale(0.95);
	    -ms-transform: scale(0.95);
	        transform: scale(0.95);
}

button:focus,
input:focus {
	color: #000;
	outline-offset: 0px;
	box-shadow: 0px 0px 0px 2px inset;
	outline:  var(--colour-focus) solid 3px;
}

button:focus {
  box-shadow: #000 0px 0px 0px 4px inset;
}

/* Don't do this */
/* button:focus {
	outline: none;
} */

button.ghost {
	background-color: transparent;
	border-color: #FFF;
}

/*****
 Forms
*****/
#signUpForm {
  display: none;
}

form {
	background-color: #FFF;
	display: -webkit-box;
	display: -ms-flexbox;
	display: flex;
	-webkit-box-align: center;
	    -ms-flex-align: center;
	        align-items: center;
	-webkit-box-pack: center;
	    -ms-flex-pack: center;
	        justify-content: center;
	-webkit-box-orient: vertical;
	-webkit-box-direction: normal;
	    -ms-flex-direction: column;
	        flex-direction: column;
	padding: 2.5rem;
	height: 100%;
	text-align: center;
}

input {
	border: 1px solid var(--colour-grey--light);
	padding: 0.8rem;
	margin-bottom: 1rem;
	width: 100%;
}

label {
  margin-bottom: 0.2rem;
  display: block;
  font-size: 0.9rem;
  font-weight: bold;
  text-align: left;
  width: 100%;
}

.container {
	background-color: #fff;
	border-radius: 10px;
  	-webkit-box-shadow: 0 14px 28px rgba(0,0,0,0.25), 
			0 10px 10px rgba(0,0,0,0.22);
  	        box-shadow: 0 14px 28px rgba(0,0,0,0.25), 
			0 10px 10px rgba(0,0,0,0.22);
	position: relative;
	overflow: hidden;
	width: 800px;
	max-width: 100%;
	min-height: 550px;
}

.form-container {
  padding: 1rem;
	position: absolute;
	top: 0;
	height: 100%;
	-webkit-transition: all 0.6s ease-in-out;
	-o-transition: all 0.6s ease-in-out;
	transition: all 0.6s ease-in-out;
}

.sign-in-container {
	left: 0;
	width: 50%;
	z-index: 2;
}

.container.right-panel-active .sign-in-container {
	-webkit-transform: translateX(100%);
	    -ms-transform: translateX(100%);
	        transform: translateX(100%);
}

.sign-up-container {
	left: 0;
	width: 50%;
	opacity: 0;
	z-index: 1;
}

.container.right-panel-active .sign-up-container {
	-webkit-transform: translateX(100%);
	    -ms-transform: translateX(100%);
	        transform: translateX(100%);
	opacity: 1;
	z-index: 5;
	-webkit-animation: show 0.6s;
	        animation: show 0.6s;
}

@-webkit-keyframes show {
	0%, 49.99% {
		opacity: 0;
		z-index: 1;
	}
	
	50%, 100% {
		opacity: 1;
		z-index: 5;
	}
}

@keyframes show {
	0%, 49.99% {
		opacity: 0;
		z-index: 1;
	}
	
	50%, 100% {
		opacity: 1;
		z-index: 5;
	}
}

.overlay-container {
	position: absolute;
	top: 0;
	left: 50%;
	width: 50%;
	height: 100%;
	overflow: hidden;
	-webkit-transition: -webkit-transform 0.6s ease-in-out;
	transition: -webkit-transform 0.6s ease-in-out;
	-o-transition: transform 0.6s ease-in-out;
	transition: transform 0.6s ease-in-out;
	transition: transform 0.6s ease-in-out, -webkit-transform 0.6s ease-in-out;
	z-index: 100;
}

.container.right-panel-active .overlay-container{
	-webkit-transform: translateX(-100%);
	    -ms-transform: translateX(-100%);
	        transform: translateX(-100%);
}

.overlay {
	background: #FF416C;
	background: -webkit-gradient(linear, left top, right top, from(#FF4B2B), to(var(--colour-primary)));
	background: -o-linear-gradient(left, #FF4B2B, var(--colour-primary));
	background: linear-gradient(to right, #FF4B2B, var(--colour-primary));
	background-repeat: no-repeat;
	background-size: cover;
	background-position: 0 0;
	color: #FFF;
	position: relative;
	left: -100%;
	height: 100%;
	width: 200%;
  	-webkit-transform: translateX(0);
  	    -ms-transform: translateX(0);
  	        transform: translateX(0);
	-webkit-transition: -webkit-transform 0.6s ease-in-out;
	transition: -webkit-transform 0.6s ease-in-out;
	-o-transition: transform 0.6s ease-in-out;
	transition: transform 0.6s ease-in-out;
	transition: transform 0.6s ease-in-out, -webkit-transform 0.6s ease-in-out;
}

.container.right-panel-active .overlay {
  	-webkit-transform: translateX(50%);
  	    -ms-transform: translateX(50%);
  	        transform: translateX(50%);
}

.overlay-panel {
	position: absolute;
	display: -webkit-box;
	display: -ms-flexbox;
	display: flex;
	-webkit-box-align: center;
	    -ms-flex-align: center;
	        align-items: center;
	-webkit-box-pack: center;
	    -ms-flex-pack: center;
	        justify-content: center;
	-webkit-box-orient: vertical;
	-webkit-box-direction: normal;
	    -ms-flex-direction: column;
	        flex-direction: column;
	padding: 0 40px;
	text-align: center;
	top: 0;
	height: 100%;
	width: 50%;
	-webkit-transform: translateX(0);
	    -ms-transform: translateX(0);
	        transform: translateX(0);
	-webkit-transition: -webkit-transform 0.6s ease-in-out;
	transition: -webkit-transform 0.6s ease-in-out;
	-o-transition: transform 0.6s ease-in-out;
	transition: transform 0.6s ease-in-out;
	transition: transform 0.6s ease-in-out, -webkit-transform 0.6s ease-in-out;
}

.overlay-left {
	-webkit-transform: translateX(-20%);
	    -ms-transform: translateX(-20%);
	        transform: translateX(-20%);
}

.container.right-panel-active .overlay-left {
	-webkit-transform: translateX(0);
	    -ms-transform: translateX(0);
	        transform: translateX(0);
}

.overlay-right {
	right: 0;
	-webkit-transform: translateX(0);
	    -ms-transform: translateX(0);
	        transform: translateX(0);
}

.container.right-panel-active .overlay-right {
	-webkit-transform: translateX(20%);
	    -ms-transform: translateX(20%);
	        transform: translateX(20%);
}

footer {
    background-color: var(--colour-grey);
    color: #fff;
    font-size: 14px;
    bottom: 0;
    position: fixed;
    left: 0;
    right: 0;
    text-align: center;
    z-index: 999;
}

footer a {
	color: #fff;
}
!

JS 

              
                const signUpButton = document.getElementById("signUp");
const signInButton = document.getElementById("signIn");
const container = document.getElementById("container");
const signUpForm = document.getElementById("signUpForm");
const signInForm = document.getElementById("signInForm");

signUpButton.addEventListener("click", () => {
	const socialSignUp = document.getElementById("socialSignUp");
	container.classList.toggle("right-panel-active");
	
	// 1. Make the sign up form visible/focusable
	// i.e. put it back in the DOM to be accessed by
	// screen readers
	signUpForm.style.display = "flex";

	// 2. Take the singIn out of index and put the signUp in
	signInButton.tabIndex = 0;
	signUpButton.tabIndex = -1;
	
	// 3. Pull focus on the other form's first social link
	socialSignUp.focus();

	  // display cannot be animated
	  // the timeout workaround enables the animation to run
	setTimeout(function() {
		signInForm.style.display = "none";	
	}, 300);
});

signInButton.addEventListener("click", () => {
	const socialSignIn = document.getElementById("socialSignIn");

	container.classList.toggle("right-panel-active");
	signInForm.style.display = "flex";
	signInButton.tabIndex = -1;
	signUpButton.tabIndex = 0;
	socialSignIn.focus();

	setTimeout(function() {
		signUpForm.style.display = "none";
	}, 300);
});

const forms = document.querySelectorAll('form'),
    fn      = function(e) { e.preventDefault() };

for ( var i = forms.length; i--; ) {
    forms[i].addEventListener('click', fn, false);
}
!
999px

Console