I have a dexterity based content type with different edit views and fieldsets defined in the schema. Whenever I want to update a widget/field in another than the first fieldset, it gets crumpy. For example:
self.groups[0].fields['my_field'].mode = HIDDEN_MODE
must be used to hide my_field from the 2nd fieldset. To make the same field required
self.schema['my_field'].required = True
does the trick.
But how can I add a css class to a widget on another than the first fieldset? I mean, what
self.fields['my_field_on_first_fieldset'].widget.addClass("myClass")
would do, if my widget would be on the first fieldset "page".
Following Martin Aspeli's tutorial, I tested your request and can confirm that oddly the klass-attribute cannot be applied on invisible widgets in the sub-fieldsets, returning a 'Key Error' in reference to the widget's name.
With other attributes, f.e. 'size', there is no problem.
Instead you can apply the class via Javascript.
If you want to provide a fallback in case the client hasn't Javascript enabled, you can add a condition in updateWidget(), checking if it is enabled and if not, apply the class with addClass() respectively the klass-attribute. Which will work then, because all fields are shown and not hidden anymore.