To create a login form using CSS, you can use HTML to structure the form and CSS to style it.

Here’s an example of how you might structure a login form using HTML:

<form action="/login" method="post">
    <label for="username">Username:</label>
    <input type="text" id="username" name="username">
    <br>
    <label for="password">Password:</label>
    <input type="password" id="password" name="password">
    <br>
    <input type="submit" value="Log In">
</form>

This creates a simple login form with a text input for the username, a password input for the password, and a submit button.

To style the form using CSS, you can use selectors to target specific elements and apply styles to them. For example:

form {
    width: 300px;
    margin: 0 auto;
    text-align: center;
    border: 1px solid #ccc;
    border-radius: 5px;
    padding: 20px;
    box-sizing: border-box;
}

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

input[type="text"], input[type="password"] {
    width: 100%;
    padding: 12px 20px;
    margin: 8px 0;
    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;
}

.error {
    color: red;
    margin-bottom: 20px;
}