The magic values AttributeModifier.VALUELESS_ATTRIBUTE_REMOVE and AttributeModifier.VALUELESS_ATTRIBUTE_ADD do not work correctly after they have been deserialized.
Imagine you have a page with a component that has sth. added like this:
Once we navigate away from the page, it gets serialized including the String value "VA_ADD" for AttributeModifier.VALUELESS_ATTRIBUTE_ADD. When we go back (e.g. with browser back button), it is deserialized. But then the deserialized String "VA_ADD" is not anymore the same as AttributeModifier.VALUELESS_ATTRIBUTE_ADD, so we have an attribute added to our component with value "VA_ADD".
I'm not sure how this can be resolved or if it can be resolved at all. My workaround is to use an AbstractReadOnlyModel that always returns AttributeModifier.VALUELESS_ATTRIBUTE_ADD, so that special value is not serialized at all.
If it's not possible to manage the deserialisation, then there should be at least some documentation about this problem and maybe convenience code using my workaround with AbstractReadOnlyModel could be provided.