A worksheet-level (and workbook-level) default style, mapped to OOXML defaulting

Cluster: styles

Scenario

A user building a worksheet with many columns wants to set a base look (font, fill, alignment, number format, border) once, at the worksheet or workbook level, rather than applying the same style object to every column or cell. Today a uniform look across a large sheet means either looping over all columns and cells to assign the identical style or reaching for OOXML's default column and row formatting, which the API does not expose ergonomically. A default-style facility would let authors declare "unless overridden, cells in this sheet use this style" and then specify only the deviations.

Spec note, not a corpus case: this is a missing authoring surface, not a current-behavior bug. It is the umbrella over two facet-specific notes already recorded, a default font (default-font-workbook-worksheet-level) and default cell protection (worksheet-default-cell-protection-unlock), generalizing the same defaulting mechanism to the whole style surface. It becomes corpus-covered once the surface exists and a case can assert a default-styled sheet renders correctly with no per-cell style stamping.

Desired behavior

• Declare a default style once. Expose a way to set a default style (font, fill, alignment, numFmt, border) scoped to a worksheet, applied as the base for every cell, column and row that does not specify its own. Cell- and column-level styles override the worksheet default, and the worksheet default overrides any workbook-wide default. This subsumes the existing default-font ask and the default-protection ask as facets of one mechanism.

• Map to OOXML defaulting, do not materialize per cell. The format already models this: the cellXfs index 0 plus the "Normal" cell style is the document-wide default; <sheetFormatPr> carries default row height and base column width; and <cols> <col> spans carry a default style and width. A well-designed API surfaces a workbook-level default feeding the Normal style and default cellXf, and a worksheet-level default mapping to <col> default spans and <sheetFormatPr>. So the feature is largely a mapping onto existing defaulting rules, not stamping the same style id onto every cell. Writing a default-styled sheet must stay O(deviations), not O(cells): a worksheet whose default is a custom font must not emit a per-cell style reference for every cell that merely inherits it. It shares the size contract of worksheet-default-cell-protection-unlock.

• Round-trip stability. Reading a file that uses a non-default base style surfaces it through the same API used to set it, so read-modify-write preserves the default rather than flattening it into explicit per-cell styles.

Open questions

• Granularity: worksheet-level only, or also a workbook-level default? The request is phrased at worksheet level, but the underlying Normal-style default is workbook-wide, so likely both, with worksheet overriding workbook.
• Which facets are defaultable at each level? Is a default border or fill meaningful at the <col> and <sheetFormatPr> level, or only font, numFmt and alignment, with fill and border requiring a real per-column style span?
• Precedence order when workbook default, worksheet default, column style, row style, and cell style all touch the same facet: define and lock the resolution chain.
• How this composes with value-based style dedup (style-dedup-value-based-and-cell-add-style), where the default is the base that per-cell deviations are diffed against.

Related: default-font-workbook-worksheet-level, worksheet-default-cell-protection-unlock, style-dedup-value-based-and-cell-add-style, set-style-over-cell-range, column-declared-numfmt-reaches-cells, default-font-must-not-be-assumed-for-column-widths.