This modified content model is valid in XHTML Content Documents. (idpf.org)
' It must support visual rendering of XHTML Content Documents as defined in EPUB Style Sheets — Reading System Conformance. (idpf.org)
As XHTML Content Documents, Navigation Documents also provide a flexible means of tailoring the navigation display using CSS and the hidden attribute [Packages 3.1] while not impacting access to information for accessible Reading Systems. (idpf.org)