Section 5.4.1 currently reads:

When a nav element carries the epub:type attribute [Content Docs 3.1] in an EPUB Navigation Document, this specification restricts the content model of the element and its descendants as follows:

http://www.idpf.org/epub/31/spec/epub-packages.html#sec-package-nav-def-model

But then in 5.4.2.5 it says:

EPUB Navigation Documents MAY include one or more nav elements in addition to the toc, page-list and landmarks nav elements defined in the preceding sections. These additional nav elements SHOULD have an epub:type attribute to provide a machine-readable semantic, and MUST have a human-readable heading as their first child.

http://www.idpf.org/epub/31/spec/epub-packages.html#sec-package-nav-def-types-other

The problem is that if you omit epub:type to avoid the content model restrictions, you're supposed to get a warning. For most people, that will mean the content model restrictions cannot be avoided.

The navigation document is also supposed to be a content document that allows any kind of additional content within it, but these requirements prevent that from truly happening.

Since machine-processing of non-standardized nav elements seems unlikely, can we drop the "should" requirement from other navs and instead shift it so that reading systems should ignore nav elements that don't have an epub:type element or carry a semantic they don't recognize? (Somewhat overlapping here with #975.)