Structural Navigation in EPUB publications 1.3

Approved May 8, 2021 by the Technical Committee of Japan DAISY Consortium

1. Introduction

This short document is our attempt to describe navigation operations in EPUB3.

2. Definitions

current-content-document

the content document being rendered.

current-block-element

that block element in the current content document which is being rendered.

current-spine-item

that itemref element in the spine item which references the current content document.

current-element-in-navdoc

that li element in the navigation document which references the current-content-document and further references the current-block-element*

current-par-node

that par element in the MO document which references the current-content-document and the current-block-element

3. Navigation within a single content document

1) Move to the next (or previous) block element

• Let current-block-element be the block element that immediately follows (resp. precedes) the current-block-element.
• Reset current-element-in-navdoc and current-par-node accordingly.

4. Spine-based navigation

1) Move to the next (or previous) spine item

• Let current-content-document be the content document that immediately follows (resp. precedes) the current-content-document.
• Let current-block-element be the first block element in the current content document.
• Reset current-element-in-navdoc and current-par-node accordingly.

5. Navdoc-based navigation

1) Move from an li element in the navigation document to the referenced content document

• Let current-content-document and current-block-element be the content document and the block element (if non, the first block element) that are referenced by the current-element-in-navdoc.

2) Move to the next (or previous) li element

• Let current-element-in­-navdoc be that li element in the navdoc which immediately follows (resp. precedes) the current-element-in­-navdoc.
• Reset current-block-element, current-content-document, and current-par-node accordingly.

3) Move to the nearest-ancestor li element

• Let current-element-in­-navdoc be that li element in the navdoc which is the nearest ancestor of the current-element-in-navdoc.
• Reset current-block-element, current-content-document, and current-par-node accordingly.

6. MO-based navigation

1) Move to the next (resp. previous) par element

• Let current-par-node be that par element in the MO document which immediately follows (resp. precedes) the current-par-node.
• Reset current-block-element, current-content-document, and current-element-in­-navdoc accordingly.

7. Desirable Consistencies

1) Same Directions

• Next in MO-based navigation and Next in

2) Reachability

• Content documents reachable by spine-based navigation does not have to be reachable by nav-doc-based navigation
• Content documents reachable by nav-doc-based navigation MUST be reachable by spine-based navigation.
• Block elements reachable by content-based MO-based navigation should be reachable
• What is reachable by MO-based navigation should be reachable by content-based