# ADR 0014: Direction For A Markdown / HTML Export Backend

## Status

Proposed — **design-only**. This ADR fixes the *direction* and the key open
decision for a future markdown/HTML export target; it does **not** implement a
backend. No code or schema changes accompany it. (The 恒一小奥 lesson ships with
`student`/`teacher` PDF targets and a `kendoku`-generated `interactives` target;
a markdown/HTML analog is future work gated on the decision below.)

Builds on **ADR-0009** (export targets are builds producing typed artifacts;
"which markdown dialect … stays open") and **ADR-0011** (presentation lives in
the template; the manifest is injected). Relates to **ADR-0013** (a non-typst
backend could ride a shell step or earn a typed step — still open).

## Context

The third-party 恒一小奥 deliverable was a slide-deck markdown bundle
(`lesson_slides.md`). A recurring goal is to export an analog of such
markdown/HTML from the same engineering file. The hard part is **math**: a
curriculum is formula-heavy, and a markdown/HTML export that turns formulas into
images (losing selectability, accessibility, and editability) is a regression.
Three facts were established (web research + reading the pinned deps), and a
fourth is the user's framing:

1. **typlite (tinymist's markdown export)** renders math **as images** and loses
   cross-references and styles — it goes through HTML as an intermediary. This is
   exactly the failure mode to avoid for a math-rich curriculum.
2. **typst 0.15 native HTML export** (the version this repo pins; `typst-html
   0.15` is already in `Cargo.lock`) emits equations as **MathML**. MathML is
   *accessibility-oriented*: selectable and screen-readable, but **not** trivially
   convertible to KaTeX/LaTeX, and rendering fidelity varies by browser.
3. **`mitex`** converts **LaTeX → typst** (an authoring-time import direction),
   **not** typst → LaTeX. So it is not, by itself, the typst-source → markdown
   lever; the user's instinct that "keep the formula source span and convert it"
   is right about *what* we want, but `mitex`'s direction is the reverse of what a
   typst→markdown path needs.
4. **The user's framing**: the content a markdown/HTML target needs and the
   content the PDF 讲义/教案 need **do not fully overlap** — so whether a
   markdown/HTML export must reuse the *same* `typst-content` source, or instead
   draw on target-specific content, is **genuinely open** and is the pivotal
   design decision here.

## Decision (direction, not implementation)

### The pivotal open decision: shared source vs. target-specific content

Before any backend is built, decide: **does the markdown/HTML target render the
same per-element `typst-content` the PDF targets render, or does it consume
target-specific content?** This ADR does **not** pre-pick it; it names it as the
gating decision and frames the two routes so the next round can choose
deliberately (constitution rule 2 — surface, do not assume):

- **(R1) Shared typst-content.** One source of truth; markdown/HTML is another
  projection of the same `.typ`. Requires a real typst-source → markdown/HTML
  converter that preserves math as editable markup. Highest reuse, hardest math
  story.
- **(R2) Target-specific content.** The element carries (or the target supplies)
  content authored for the web medium. Decouples the hard typst→markdown math
  problem from the PDF path, at the cost of a second content surface to maintain.

### Preferred math direction (whichever route is chosen)

Math must survive as **markup, not images**. The favored approach is to obtain
each equation's **source span** from the typst AST and convert that span's source
to LaTeX/KaTeX, rather than accept typlite's image rendering or 0.15's
accessibility-MathML as the final form. Concretely, the next round should
evaluate:

- harvesting equation source spans via `typst-syntax` and emitting
  `$…$`/`$$…$$` (KaTeX-compatible) from the typst math source, and
- studying tinymist's own conversion crate (typlite / `cmarker`) for reusable
  pieces — adopting what helps, but **not** its math-as-image fallback.

A small **proof-of-concept** scoped to *this* lesson (whose math is trivial —
`8×`, `2-`, `12×`) is the right first experiment: it exercises the pipeline
end-to-end while sidestepping the hardest formula cases, and shows whether
equations come out as text/KaTeX rather than images.

### Backend mechanics, once the route is chosen

- typst 0.15 HTML export is reachable today (`typst-html 0.15` is a transitive
  dep; promotable to a direct dep) — usable for R1's HTML side.
- A markdown (rather than HTML) artifact may post-process that HTML, or emit
  markdown directly from the AST — open, decided with the route.
- Per ADR-0013, the backend may run as a typed step or a shell step; whether the
  non-typst backend earns its own typed `Step` constructor is still open.

## Consequences

- The export-backend work has an explicit gating decision (R1 vs. R2) and a
  pinned math principle (markup, not images), so the next round starts from a
  decision rather than a blank page.
- No implementation, schema, or Lean change lands now — the contract is
  unchanged; this is a recorded design direction.
- The PDF and interactives paths are unaffected and remain the lesson's shipping
  targets.

## Open Questions / Deferred

- **R1 vs. R2** — the pivotal decision above; unresolved by design.
- **Concrete math conversion** — span-harvest → KaTeX vs. reusing a tinymist
  crate; to be settled by the PoC.
- **Markdown dialect** — which dialect/fenced-div conventions the markdown
  artifact targets (ADR-0009 left this open; still open).
- **Typed non-typst step** — whether the backend rides `Shell` or earns a typed
  `Step` (ADR-0013 open question).
