A document-wide default font must be settable once, per workbook (and per worksheet)

Cluster: styles

Resolved at the workbook level; the worksheet level is deliberately deferred. Workbook now carries setDefaultFont / defaultFont / declaredDefaultFont, the writer emits the resolved font as styles-part font 0, and the theme's body face reaches it by derivation. See ADR-0025 for the precedence chain and the reasoning, and the workbook-default-font-is-declared-not-assumed corpus case for the locked behaviour. The scenario below turned out to describe a correctness bug rather than only an ergonomics gap: the writer spliced a Calibri constant into <fonts> with no workbook input, so a file's own font 0 was replaced on every save. That half is the sibling note, default-font-must-not-be-assumed-for-column-widths.

Every open question below is answered in ADR-0025 except the worksheet-level override, which stays open by choice: OOXML has no per-sheet default font, so it can only be sugar that stamps every column, and nothing about the workbook level constrains how that sugar should look.

Scenario

A user building a workbook wants every cell to inherit a chosen base font (family, size, color) instead of the hard-coded Calibri 11pt default, without setting the font on every row, column, or cell. They expect to declare a default font once, per workbook, with an optional per-worksheet override, and have empty and populated cells alike render in it. A correctness symptom motivates this too: when the written package carries an incomplete or missing default font entry in the styles part, some applications (Apple Numbers, and Excel in some cases) warn "missing default font" on open, because empty cells fall back to a default the styles part does not properly define.

Desired behavior

• Expose a first-class way to set the document's default font once, rather than forcing per-cell, per-row or per-column styling. Two natural levels:
  • Workbook-level default font, the base font applied to all sheets unless overridden. This is the first entry of the styles part's font table, referenced by the "Normal" cell style; changing it changes the rendered font of every cell with no explicit font override, including empty cells.
  • Optional worksheet-level override, for example a default or style option when adding a worksheet, shaped like addWorksheet(name, { style: { font: { size, name } } }).
• Correctness invariant to lock once implemented: the written styles part always contains a well-formed default font entry (name, size, family, color/theme, scheme) referenced by the base cell style, so foreign readers do not warn about a missing default font for empty cells. This holds for the built-in Calibri 11 default and especially for a custom default.
• Round-trip: reading a file with a non-Calibri default font surfaces that default through the same API used to set it, so read-modify-write preserves it.

Prior art / workarounds

• Post-processing every cell after writing to stamp font.size and font.name on any missing them, which is O(cells) and misses truly-empty cells unless iterating with includeEmpty.
• Monkey-patching the styles serializer to inject a font entry, which is fragile and reaches into internals.
• Wrapping addRow to stamp a default font on each new row.

All are brute-force symptoms of there being no supported knob for the base font.

Open questions

• Precedence and merge: workbook default against worksheet default against column/row default against explicit cell font. Confirm the inheritance chain, and whether partial fonts merge (set only size, keep default name) or replace wholesale.
• Does setting a workbook default rewrite the theme's minor and major font, or only the styles font table? This affects whether Excel labels it a theme font or a direct font.
• Interaction with the shared-strings and inline default, and with rich-text runs carrying their own fonts.

Related: default-font-must-not-be-assumed-for-column-widths (the geometry face of the same default-font question), theme-color-font-backed-by-theme-part, template-styles-survive-read-write-roundtrip.