[DanielNg0729]

Hi maintainers, this PR adds a way to declare vertical layout (#169). An example is Japanese or Chinese text in vertical lines that follow each other from right to left. This is a first proposal, so I am happy to change the name, the values, or the place of the element.

Proposal

A new empty element, <writing_mode value="..."/>, follows the pattern of <layer>:

<doclang>
  <head>
    <writing_mode value="vertical_rl"/>          <!-- document default -->
  </head>
  <heading>第一章</heading>
  <text>吾輩は猫である。名前はまだ無い。</text>
  <page_footer>
    <writing_mode value="horizontal_tb"/>        <!-- override for this element -->
    12
  </page_footer>
</doclang>

• Values: horizontal_tb, vertical_rl and vertical_lr. They correspond to the CSS writing-mode values horizontal-tb, vertical-rl and vertical-lr, written in snake_case like read_only.
• value is required. An element without <writing_mode> already inherits a writing mode, so an empty <writing_mode/> would have no use.
• In <head>: the element sets the document default. Schematron allows at most one there.
• In the element head: the element comes after <layer> and before <location>. It applies to the element and its descendants. List items and table cells share the element head, so they can use it too, for example for a vertical header cell in a horizontal table.
• Inheritance: an element without <writing_mode> takes the writing mode of its nearest ancestor that declares one. Then it takes the document default, and then horizontal_tb.
• What does not change:
  • The content stays in reading order.
  • <location> keeps its coordinate system (the top-left corner of the page).
  • The element is independent of <rtl>, which still marks right-to-left text in a line. CSS also keeps writing-mode and direction separate.
• Later: Annex D lists finer vertical typography as a possible extension: upright or rotated Latin, tate-chū-yoko, and ruby.

Changes

• doclang/doclang.xsd: a new writing_mode element, added to the element_head group after layer. The global declaration also validates the element inside the lax <head>.
• doclang/doclang.sch:
  • writing_mode is added to every list of element-head names: element-head placement, list, track, table, picture body, list items, and table cells.
  • A new pattern, head-writing-mode-unique, allows at most one writing_mode in <head>.
• spec.md:
  • The Element Head list includes the new element.
  • A new "Writing Mode" usage section has three examples.
  • The token vocabulary has a new row for <writing_mode value=".
  • Annex D has a new bullet, and the Bibliography has a new entry for CSS Writing Modes Level 3.
  • Annex A is regenerated from the workbook.
• reference/input/reference.xlsx: one new elements row and one new attributes row. No existing row changes.
• doclang/tokenization.py: a new token, '<writing_mode value="'.
• doclang/migration/_steps/next.py: the migration step removes writing_mode and records a "dropped" change for each element.
• Test fixtures:
  • 3 valid fixtures: a head default with overrides, the full element-head order, and a vertical table and list with cell and item overrides.
  • 8 invalid fixtures: a wrong value, a missing value, a wrong value in <head>, after <location>, before <layer>, in the body, directly in <doclang>, and twice in <head>.
• tests/test_migration.py: a new test for the dropped elements and their reported locations.
• tests/test_validation.py:
  • test_newer_declared_version_falls_back_to_the_released_head expected the head schema to be identical to the released 0.8 schema. This stops being true with any head change.
  • The test now expects "(unreleased development version)", and its new name is test_newer_declared_version_falls_back_to_the_head.

Issue resolved by this Pull Request:
Resolves #169

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.