I've found the fix for this item.In this file /include/class.format.php lines 234 - 244 there is some code the unsets unsupported classes.// Clean unexpected class values if (isset($attributes)) { $classes = explode(' ', $attributes); foreach ($classes as $i=>$a) // Unset all unsupported style classes -- anything but M$ if (strpos($a, 'Mso') !== 0) unset($classes); if ($classes) $attributes = implode(' ', $classes); else unset($attributes); }All I needed to do was change the class name I used in my html to start with "Mso". Dont quite understand why they need to start with "Mso" but now my class is not removed from my FAQ. <div class="Mso-embed-container"><iframe src='https://player.vimeo.com/video/1234567' frameborder='0' webkitAllowFullScreen mozallowfullscreen allowFullScreen></iframe></div>