In the property panel the class can be set on all kind of objects. The Css class can only be found in the css-files. In the modeler there is no feedback if this class is allowed or not. Also when there is a typo made the user will not get any feedback. It would be very convenient if the user gets feedback to see the available classes, and that the user can select the right class. Example solution would be:
this is pretty tricky.
I think design properties are pretty close to solving this, the issue is that a css class can be really complex, have multiple rules the over rule themselves not to mention responsiveness.
I would recommend talking to your Ux designer, and looking into the .cls tool in chrome, to see the available classes.