When creating a quiz intro that is simply HTML (in my example it was a table with images (no text)) the intro is not displayed. However, when editing the HTML and adding ASCII characters it did. This occurs because of the if statement in mod/quiz/view.php (if (trim(strip_tags($quiz->intro)))). I am not sure if this is a bug or expected behaviour, but it does mean that the user is forced to enter text, but are not told this is necessary. Changing the if statement to if (!empty($quiz->intro)) solves this issue, but may introduce vulnerabilities. However, the intro is cleaned in the if statement anyways.