glide.ui.html.editor.extended_valid_elements

ServiceNow property description:

Defines which elements will remain in the Tiny MCE text when the editor saves. This can be very useful to add or override specific elements that should be kept. When adding a new attribute by specifying an existing element rule (e.g. img), the entire rule for that element is over-ridden so be sure to include all valid attributes not just the one you wish to add or override. See http://archive.tinymce.com/wiki.php/Configuration:extended_valid_elements