Webleb - HTML UI Elements -

<!DOCTYPE html>
<html lang="es" >
<head>
 <meta charset="UTF-8">
 <title>Andev web</title>
 <link rel="stylesheet" href="./style.css">

</head>
<body>
<section>

<div class="signin">
 <div class="content">
 <h2>Inicio</h2>
 <div class="form">
 <div class="inputBox">
 <input type="text" required>
 Usuario
 </div>
 <div class="inputBox">
 <input type="password" required>
 Contraseña
 </div>
 <div class="links">
 <a href="#">Olvidé la Contraseña</a>
 <a href="#">Registrase</a>
 </div>
 <div class="inputBox">
 <input type="submit" value="Iniciar">
 </div>
 </div>
 </div>
 </div>
 </section>
 
</body>
</html>

@import url('https://fonts.googleapis.com/css2?family=Quicksand:wght@300;400;500;600;700&display=swap');
*
{
	margin: 0;
	padding: 0;
	box-sizing: border-box;
	font-family: 'Quicksand', sans-serif;
}
body 
{
	display: flex;
	justify-content: center;
	align-items: center;
	min-height: 100vh;
	background: #000;
}
section 
{
	position: absolute;
	width: 100vw;
	height: 100vh;
	display: flex;
	justify-content: center;
	align-items: center;
	gap: 2px;
	flex-wrap: wrap;
	overflow: hidden;
}
section::before 
{
	content: '';
	position: absolute;
	width: 100%;
	height: 100%;
	background: linear-gradient(#000,#0f0,#000);
	animation: animate 5s linear infinite;
}
@keyframes animate 
{
	0%
	{
		transform: translateY(-100%);
	}
	100%
	{
		transform: translateY(100%);
	}
}
section span 
{
	position: relative;
	display: block;
	width: calc(6.25vw - 2px);
	height: calc(6.25vw - 2px);
	background: #181818;
	z-index: 2;
	transition: 1.5s;
}
section span:hover 
{
	background: #0f0;
	transition: 0s;
}

section .signin
{
	position: absolute;
	width: 400px;
  background: #222;  
	z-index: 1000;
	display: flex;
	justify-content: center;
	align-items: center;
	padding: 40px;
	border-radius: 4px;
	box-shadow: 0 15px 35px rgba(0,0,0,9);
}
section .signin .content 
{
	position: relative;
	width: 100%;
	display: flex;
	justify-content: center;
	align-items: center;
	flex-direction: column;
	gap: 40px;
}
section .signin .content h2 
{
	font-size: 2em;
	color: #0f0;
	text-transform: uppercase;
}
section .signin .content .form 
{
	width: 100%;
	display: flex;
	flex-direction: column;
	gap: 25px;
}
section .signin .content .form .inputBox
{
	position: relative;
	width: 100%;
}
section .signin .content .form .inputBox input 
{
	position: relative;
	width: 100%;
	background: #333;
	border: none;
	outline: none;
	padding: 25px 10px 7.5px;
	border-radius: 4px;
	color: #fff;
	font-weight: 500;
	font-size: 1em;
}
section .signin .content .form .inputBox i 
{
	position: absolute;
	left: 0;
	padding: 15px 10px;
	font-style: normal;
	color: #aaa;
	transition: 0.5s;
	pointer-events: none;
}
.signin .content .form .inputBox input:focus ~ i,
.signin .content .form .inputBox input:valid ~ i
{
	transform: translateY(-7.5px);
	font-size: 0.8em;
	color: #fff;
}
.signin .content .form .links 
{
	position: relative;
	width: 100%;
	display: flex;
	justify-content: space-between;
}
.signin .content .form .links a 
{
	color: #fff;
	text-decoration: none;
}
.signin .content .form .links a:nth-child(2)
{
	color: #0f0;
	font-weight: 600;
}
.signin .content .form .inputBox input[type="submit"]
{
	padding: 10px;
	background: #0f0;
	color: #000;
	font-weight: 600;
	font-size: 1.35em;
	letter-spacing: 0.05em;
	cursor: pointer;
}
input[type="submit"]:active
{
	opacity: 0.6;
}
@media (max-width: 900px)
{
	section span 
	{
		width: calc(10vw - 2px);
		height: calc(10vw - 2px);
	}
}
@media (max-width: 600px)
{
	section span 
	{
		width: calc(20vw - 2px);
		height: calc(20vw - 2px);
	}
}

Validating your code, please wait...

HTML

CSS

<!DOCTYPE html>
<html lang="es" >
<head>
 <meta charset="UTF-8">
 <title>Andev web</title>
 <link rel="stylesheet" href="./style.css">

</head>
<body>
<section>