Compatibility boundary

SheetMill implements its own limited source contract inspired by Markdown reference card conventions. This is not full compatibility with Fechin/reference, its Hexo plugins, Tailwind classes or theme layout. GFM support comes from Goldmark; compatibility means preserving document semantics in the supported subset.

Accepted headings, spans, IDs and inert .wrap are specified in gfm-plus.md. The semantic classes .note, .tip, .warning and reserved .navi belong to SheetMill's contract, without an upstream Reference compatibility guarantee. Unknown heading attributes are errors in v0.1; the controlled Reference adapter reports supported conversions and rejects unsupported structural attrs explicitly.

testdata/reference-compatible/ contains independently authored examples, not copied upstream sheets. Fixtures cover ordinary content, tables/alignment, task lists, links, nested lists/quotes/headings, longer code fences, literal shorter fences, layout hints, unknown metadata and invalid conflicts. Each valid fixture has a checked-in JSON golden snapshot. Snapshot tests also compare repeated parses.

No upstream GPL-3.0 implementation, CSS, plugin or authored sheet was copied. Renderer layout and code-export behavior are outside this milestone.