JSFiddle - React, Tailwind, and code Playground

HTML

<div id="divRegistrazione">
<form method="post" action="Registrazione_action.php">
<table id="tableRegistrazione">
<tr><td class="label"><label form="tableRegistrazione" for="username">Username:</label></td><td class="input"><input type="text" name="username" required /></td></tr>
<tr><td class="label"><label form="tableRegistrazione" for="mail">Mail:</label></td><td class="input"><input type="email" name="mail" required /></td></tr>
<tr><td class="label"><label form="tableRegistrazione" for="password">Password:</label></td><td class="input"><input type="password" name="password" required /></td></tr>
<tr><td class="label"><label form="tableRegistrazione" for="confermaPassword">Conferma password:</label></td><td class="input"><input type="password" name="confermaPassword" required /></td></tr>
</table><br><script type="text/javascript" src="http://www.google.com/recaptcha/api/challenge?k=6Lc4C-ISAAAAAJW5DPV9TVS2q_m0PMjMBpS6wxYi"></script>

	<noscript>
  		<iframe src="http://www.google.com/recaptcha/api/noscript?k=6Lc4C-ISAAAAAJW5DPV9TVS2q_m0PMjMBpS6wxYi" height="300" width="500" frameborder="0"></iframe><br/>
  		<textarea name="recaptcha_challenge_field" rows="3" cols="40"></textarea>
  		<input type="hidden" name="recaptcha_response_field" value="manual_challenge"/>
	</noscript><br>
<input type="submit" value="Registrati" />
</form>
</div>

CSS

body {
	font-family: Arial, sans-serif;
}
#container {
	width: 100%;
	position: absolute;
	top: 0px;
	left: 0px;
}

#header {
	background-color: #DBDBFF;
}
#tableHeader {
	width: 100%;
}

#header, #divBody {
	padding: 10px;
}

footer {
	text-align: center;
}

footer a {
	color: black;
	text-decoration: none;
}

table {
	text-align: center;
	margin-left: auto;
	margin-right: auto;
	width: 85%;
}

#tableLogin {
	text-align: left;
}

.label {
	color: gray;
	text-align: right;
}

.input {
	text-align: left;
}

.label .input {
	width: 50%;
}

#divRegistrazione {
	text-align: center;
}

#recaptcha_area, #recaptcha_table {
    margin: auto;
}