Following hyperlinks to non-linear spine items should be a normative statement #1864

https://w3c.github.io/epub-specs/epub33/rs/#sec-pkg-doc-spine says:
"When a user traverses the default reading order defined in the spine [EPUB-33], a Reading System MAY automatically skip itemref elements marked as non-linear (excluding when a user specifically activates a hyperlink to such items)."

The parenthetical is an important requirement; IMO it should be a separate sentence with a clear MUST. Once it becomes a normative statement, it should gain a data-tests attribute pointing at https://w3c.github.io/epub-tests/#confreq-spine-nonlinear, which is the test for this requirement (currently structured as a test of https://w3c.github.io/epub-specs/epub33/core/#confreq-spine-nonlinear in the authoring spec).

Optionally, the test should also be renamed to match the ID of the new normative statement, since it's really intended to be testing that, and the authoring spec is secondary here.