Sidebar Builder

<!--Write a PHP program SidebarBuilder.php that takes data from several input fields and builds 3 sidebars.
The input fields are categories, tags and months. The first sidebar should contain a list of the categories,
the second sidebar – a list of the tags and the third should contain the months.
The entries in the input strings will be separated by a comma and space ", ".
Styling the page is optional. Semantic HTML is required.-->

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>Sidebar Builder</title>
    <style type="text/css">
        section {
                width: 60%;
                float: left;
        }

        input[type="text"] {
                    width: 300px;
                text-indent: 2px;
        }

        aside {
                width: 35%;
                float: right;
        }

        article {
                width: 80%;
                font-family: Tahoma, sans-serif;
                font-size: 18px;
                background-color: #FF0;
                padding: 5px 0 5px 15px;
                margin-bottom: 10px;
                border: 1px solid #000;
                border-radius: 20px;
        }

        h2 {
                border-bottom: 1px solid #000;
        }

        ul {
                list-style-type: circle;
        }

        li {
                text-decoration: underline;
        }
    </style>
</head>
<body>
<section>
    <form method="post">
        <p>
            <label for="cats">Categories:</label>
            <input type="text" name="cats" id="cats" autofocus required/>
        </p>

        <p>
            <label for="tags">Tags:</label>
            <input type="text" name="tags" id="tags" required/>
        </p>

        <p>
            <label for="months">Months:</label>
            <input type="text" name="months" id="months" required/>
        </p>
        <input type="submit" value="Generate"/>
    </form>
    <p>
        <?php
        if (!isset($_POST['cats']) || !isset($_POST['tags']) || !isset($_POST['months']) ||
            trim($_POST['cats']) === '' || trim($_POST['tags']) === '' || trim($_POST['months']) === ''
        ) {
            die('Please, enter some text in Each Text-field on Input Form!!!');
        }
        ?>
    </p>
</section>
<?php
$categories = preg_split('/,\s+/', trim($_POST['cats']), -1, PREG_SPLIT_NO_EMPTY);
$tags = preg_split('/,\s+/', trim($_POST['tags']), -1, PREG_SPLIT_NO_EMPTY);
$months = preg_split('/,\s+/', trim($_POST['months']), -1, PREG_SPLIT_NO_EMPTY);
?>
<aside>
    <article>
        <h2>Categories</h2>
        <ul>
            <?php
            foreach ($categories as $categ):?>
                <li><?= htmlspecialchars($categ) ?></li>
            <?php endforeach; ?>
        </ul>
    </article>
    <article>
        <h2>Tags</h2>
        <ul>
            <?php
            foreach ($tags as $tag):?>
                <li><?= htmlspecialchars($tag) ?></li>
            <?php endforeach; ?>
        </ul>
    </article>
    <article>
        <h2>Months</h2>
        <ul>
            <?php
            foreach ($months as $month):?>
                <li><?= htmlspecialchars($month) ?></li>
            <?php endforeach; ?>
        </ul>
    </article>
</aside>
</body>
</html>