It is possible to add custom CSS classes to HTML text (such as the Text property of the Text addon or the Content property of the Table Cells item in the Table addon) in the lesson editor using the HTML editor or the Items Editor. In order to do so, you must first define a new CSS class with an "additional_" prefix (for instance, "additional_custom-class") in the content's CSS. Once at least one such class has been added, an "Additional Class" drop-down list will appear in the HTML Editor and the Items Editor. Note that the "additional_" prefix will be omitted in the drop-down list. Finally, you should select the section of the text that you wish to apply the CSS class to and then pick that class from the "Additional Class" drop-down list.