CSS Div标签不起作用

Question

I'm trying to use a div tag to add CSS to a page that I include with include 'login.php'; inside the <div> tag but the css doesn't seem to be working. My ultimate goal is to get the form to overlay the page but right now I just want some sort of styling to work so I can go from there.

EDIT: Turns out my stylesheet doesn't register or connect to the document because it works if I use the style tags to create a stylesheet inside index.php. No idea what I did wrong, dreamweaver recognizes it.

index.php

<div id="login">
<?php
    if(isset($_GET['action']) && strcmp($_GET['action'], "login") == 0) {
        include 'Users/loginPopUp.php';
    }
?>
</div>

loginPopUp.php

<form method="post" action="">
        <ul>
            <li>
                <label for="usn">Username : </label>
                <input type="text" maxlength="30" required autofocus name="username" />
            </li>

            <li>
                <label for="passwd">Password : </label>
                <input type="password" maxlength="30" required name="password" />
            </li>
            <li class="buttons">
                <input type="submit" name="login" value="Log me in" />
                <input type="button" name="register" value="Register" onclick="location.href='register.php'" />
            </li>

        </ul>
    </form>

style.css

#login{
    background:#F00;
}

Answer 1

First, you should add the CSS File to the header of the page.

Second, once you've done that just add div="#login" in the form tag.