Here is an example of how you might create a registration form using HTML and CSS:

<form action="/register" method="post">
    <div class="form-group">
        <label for="username">Username:</label>
        <input type="text" id="username" name="username" class="form-control" required>
    </div>
    <div class="form-group">
        <label for="email">Email:</label>
        <input type="email" id="email" name="email" class="form-control" required>
    </div>
    <div class="form-group">
        <label for="password">Password:</label>
        <input type="password" id="password" name="password" class="form-control" required>
    </div>
    <div class="form-group">
        <label for="confirm-password">Confirm Password:</label>
        <input type="password" id="confirm-password" name="confirm-password" class="form-control" required>
    </div>
    <div class="form-group">
        <input type="submit" value="Sign Up" class="btn btn-primary">
    </div>
</form>

form {
    max-width: 500px;
    margin: 0 auto;
    padding: 20px;
    box-sizing: border-box;
}

.form-group {
    margin-bottom: 20px;
}

label {
    display: block;
    font-weight: bold;
    margin-bottom: 8px;
}

input[type="text"],
input[type="email"],
input[type="password"] {
    width: 100%;
    padding: 12px 20px;
    box-sizing: border-box;
    border: 1px solid #ccc;
    border-radius: 4px;
    font-size: 16px;
}

input[type="submit"] {
    width: 100%;
    background-color: #4CAF50;
    color: white;
    padding: 14px 20px;
    margin: 8px 0;
    border: none;
    border-radius: 4px;
    cursor: pointer;
    font-size: 16px;
}

input[type="submit"]:hover {
    background-color: #45a049;
}

This creates a registration form with fields for the username, email, password, and confirm password. It also includes a submit button to submit the form. The form is centered on the page and has a fixed width of 500 pixels. The input fields have a border, padding, and rounded corners, and the submit button has a hover effect.