I believe this is a bug, I was fighting it a bit today and was able to recreate it.
I have set up a class for forms and form fields, "size50, size100, etc..". When you check off to apply this to a form or a form field, it does not get added to the HTML. Now, if you enter in any value to the "CSS Class" field that is standard with Contao, it will then add the selected styles to the HTML.
I suspect whatever is being triggered to add the classes isn't being reached with a blank "CSS Class" field.
I believe this is a bug, I was fighting it a bit today and was able to recreate it.
I have set up a class for forms and form fields, "size50, size100, etc..". When you check off to apply this to a form or a form field, it does not get added to the HTML. Now, if you enter in any value to the "CSS Class" field that is standard with Contao, it will then add the selected styles to the HTML.
I suspect whatever is being triggered to add the classes isn't being reached with a blank "CSS Class" field.