Property additionalAttributes 

All Fluid ViewHelper that create exactly one HTML tag, tag-based ViewHelpers, can get passed the property additionalAttributes.

A tag-based Fluid ViewHelper generally supports most attributes that are also available in HTML. There are, for example, the attributes class and id, which exist in all tag-based ViewHelpers.

Sometimes attributes are needed that are not provided by the ViewHelper. A common example are data attributes.

<f:form.textbox additionalAttributes="{data-anything: 'some info', data-something: some.variable}" />

The property additionalAttributes is especially helpful if only a few of these additional attributes are needed. Otherwise, it is often reasonable to write an own ViewHelper which extends the corresponding ViewHelper.

The property additionalAttributes is provided by the TagBasedViewHelper so it is also available to custom ViewHelpers based on this class. See chapter Developing a custom ViewHelper.