title: Generating pdf error, although work fine on github, not in localhost
body: <p>So I have been working on card generating form html to pdf.
  I finally got my hands on a github project made by mozilla people
  <a href="https://github.com/MozillaKerala/Card-Generator" rel="nofollow noreferrer">Mozilla Kerala</a></p>
  <p>It runs ok on their given link. But, when i download it in my localhost i get something like this. when i click generate card.</p>
  <blockquote>
  <p>Warning: Illegal string offset 'ID' in C:\xampp\htdocs\demo\vendor\mpdf\classes\cssmgr.php on line 1302</p>
  <p>Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\demo\vendor\mpdf\classes\cssmgr.php on line 1302</p>
  </blockquote>
  <p>This is the line in source code of cssmgr.php.</p>
  <pre><code>    1297    $classes = array();
      1298    if (isset($attr['CLASS'])) {
      1299     $classes = preg_split('/\s+/', $attr['CLASS']);
      1300    }
      1301    if (!isset($attr['ID'])) {
      1302         $attr['ID'] = '';
      1303        }
  </code></pre>
  <p>Kindly, let me know if there any tweak to be done.
  Thanks</p>