Composes single-file HTML artifacts (PR review writeups, status reports, incident postmortems, slide decks, design systems, prototypes, flowcharts, module maps, feature explainers, kanban boards, prompt tuners) from a small JSON spec instead of hand-written HTML/CSS/JS. Use when the user asks to "compare options side-by-side", requests an HTML version of a report or review or deck, asks for a flowchart, status update, postmortem, design system reference, interactive prototype, custom editor —...
Scanned 9/4/2026
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---
name: composing-html
description: Composes single-file HTML artifacts (PR review writeups, status reports, incident postmortems, slide decks, design systems, prototypes, flowcharts, module maps, feature explainers, kanban boards, prompt tuners) from a small JSON spec instead of hand-written HTML/CSS/JS. Use when the user asks to "compare options side-by-side", requests an HTML version of a report or review or deck, asks for a flowchart, status update, postmortem, design system reference, interactive prototype, custom editor — or explicitly says "HTML artifact", "single HTML file", "self-contained HTML". Skip for ad-hoc HTML snippets (forms, emails, embedded widgets) where there's no template fit.
metadata:
version: 0.5.0
---
# composing-html
Produce single-file HTML artifacts without hand-writing the page chrome. The
composer supplies `<!DOCTYPE>`, `<head>`, inlined CSS, `base.js`, design
tokens, masthead, and colophon. You supply a title and the body content.
The product is the **chrome and inventory below** — primitives you can drop
into any artifact without re-deriving what a card, badge, or eyebrow looks
like. Templates are shortcuts on top of this, useful when the same artifact
shape repeats; see [Templates](#templates-shortcuts-for-repeat-structure)
near the end.
## Default workflow: freeform
`freeform` gives you the whole chrome with one content slot — `body_html` —
for the page body. Reach for it first. Reach for a template only when the
structure repeats across artifacts (see [Templates](#templates-shortcuts-for-repeat-structure)
near the end).
There are two ways to invoke it. **Use the `--set` flow for anything with a
substantial body** — it sidesteps the JSON-string escaping that bites
heredoc-style spec writing (newlines, quotes, `<`/`&` inside multi-line
HTML).
### Recommended: HTML in a file, metadata via `--set`
```
1. Write the body to a .html file directly (no JSON, no escaping).
2. python scripts/build.py build freeform \
--set title='My Page' \
--set subtitle='Optional subhead' \
--set body_html=@body.html \
--out artifact.html
```
`--set KEY=VALUE` assigns a literal string; `--set KEY=@FILE` loads the file
contents verbatim into that spec field. Repeat for any field. Works for
`body_html`, `extra_css`, `extra_js`, `eyebrow`, `page_class`, and the same
`*_html` fields in any other template (`summary_html`, `intro_html`,
`details_html`, …).
### Spec-file workflow (best for structured templates)
```
1. python scripts/build.py describe <template> # required keys + skeleton
2. write spec.json
3. python scripts/build.py build <template> --spec spec.json --out artifact.html
```
For templates with typed slots (`pr_review.findings[]`, `slide_deck.slides[]`,
`status_report.metrics[]`), the spec file is the right shape — the template
reasons over the structure. For `freeform`, the spec is mostly a thin config
wrapper around one HTML string; the `--set` flow above is usually less
friction.
You can mix both: small `spec.json` for metadata, `--set body_html=@body.html`
for the heavy bit. `--set` overrides any matching field from `--spec`.
### Pitfall: don't inline multi-line HTML into a JSON heredoc
`cat > spec.json <<EOF { "body_html": "<multi\nline>\n..." } EOF` does not
produce valid JSON — JSON strings can't contain raw newlines or unescaped
quotes. Either:
- use `--set body_html=@body.html` (recommended), or
- assemble the spec in Python with `json.dump(spec, f)` so escaping is automatic.
## Inventory
Everything in this section is loaded into every artifact via inlined CSS and
`base.js`. Use these tokens and classes inside `body_html` (or any
template's `*_html` field) without re-declaring them.
### Color tokens
| Token | Hex | Use |
|---|---|---|
| `--ivory` | `#FAF9F5` | Page background |
| `--paper` | `#FFFFFF` | Card background |
| `--slate` | `#141413` | Headings, inverted background |
| `--clay` | `#D97757` | Brand accent (lines, primary actions) |
| `--clay-d` | `#B85C3E` | Hover/dark variant |
| `--oat` | `#E3DACC` | Soft contrast surface |
| `--olive` | `#788C5D` | Success, secondary accent |
| `--rust` | `#B04A3F` | Errors, destructive |
| `--moss` | `#4A6B3A` | Success text |
| `--g100` … `--g700` | grays | Surfaces, borders, body text |
Semantic aliases: `--ok`, `--warn`, `--err`, `--info`.
### Type stacks
- `--serif` — display headings (h1, h2, big numerics).
- `--sans` — body text (default).
- `--mono` — code, eyebrows, badges, captions.
### Geometry
`--radius-sm` (6px) · `--radius` (10px) · `--radius-lg` (16px) ·
`--border` · `--border-soft` · `--shadow-card` · `--shadow-pop`.
### Layout primitives
- `.page` — main column (1080px max). Variants: `.page--wide` (1280px),
`.page--narrow` (720px). Set via the `page_class` spec key.
- `.masthead` — header strip with `.eyebrow` + `<h1>` + `.subtitle`
(auto-rendered from `title`/`subtitle`/`eyebrow` unless `show_masthead`
is false).
- `.grid .grid--2|3|4|auto` — responsive CSS grid.
- `.stack`, `.row` — vertical / horizontal flex.
- `.card`, `.card--soft`, `.card--elev` — content containers.
- `.rule` — `<hr>` underline below `<h2>`.
- `.colophon` — optional footer strip; pass `colophon="text"` to `page()` to
show it (off by default).
### Components
- **Eyebrow**: `<div class="eyebrow">SECTION</div>` — small all-caps label
with a leading clay rule.
- **Badge**: `<span class="badge badge--ok|warn|err|info|clay">v1.0</span>`.
- **Kbd**: `<span class="kbd">⌘K</span>`.
- **Bullets**: `<ul class="bullets"><li>…</li></ul>` — clay dots.
- **Code**: inline `<code>` and block `<pre><code>`. Block code gets a
`copy` button automatically via `base.js`.
- **Details**: native `<details><summary>…</summary>…</details>` styled.
### Tabs
```html
<div class="tabgroup">
<div class="tabs">
<button data-target="a">Tab A</button>
<button data-target="b">Tab B</button>
</div>
<div class="tab-panel" data-id="a">…</div>
<div class="tab-panel" data-id="b">…</div>
</div>
```
`base.js` wires this automatically and selects the first tab by default.
### Drag-to-reorder
```html
<div data-sortable="true">
<div draggable="true">…</div>
<div draggable="true">…</div>
</div>
```
Optional cross-zone drops: add `data-zone="<id>"` to each container.
### Live parameter bindings
```html
<input type="range" data-bind="size" min="0" max="100" value="50" data-format="number" data-unit="px">
<span data-out="size"></span>
<style>.box { width: var(--bind-size, 50px); }</style>
```
The CSS custom property `--bind-<name>` is updated on every input event,
and any `[data-out="<name>"]` element receives the formatted value.
## Output rules
Spend output tokens on **content**, not chrome:
1. **Never write `<html>`, `<head>`, `<style>`, `<script>`, or `<link>`.** The
composer adds all of them. If you find yourself writing a complete page,
you missed the skill. <!-- rule:chrome-leak -->
2. **Don't restate design tokens.** Reuse the inventory above — `var(--clay)`,
`.card`, `.badge--warn`, `.bullets`, etc. are already loaded. Don't
hardcode hex/`rgb()` colours, inline `font-family`/`font-size`, or
reference tokens that aren't in the palette.
<!-- rule:hardcoded-color rule:inline-typography rule:undefined-token -->
3. **`body_html` is HTML, not a JSON dialect.** Write `<section>`, `<h2>`,
`<ul class="bullets">` directly. No translation layer.
4. **Anything in an `_html` field is inserted verbatim** — escape any
user-supplied content yourself. All other string values are
HTML-escaped automatically.
5. **One artifact per build.** Browser tabs are free.
## Checking output
After building, lint the artifact before presenting it:
```
python scripts/build.py check artifact.html
```
The checker is deterministic — no model call, stdlib only. It doesn't grade
taste (the fixed chrome already prevents the usual AI tells); it flags content
that breaks *out* of the design system or wires `base.js` hooks to nothing —
the failure modes the chrome can't prevent on its own:
| rule | catches | severity |
|---|---|---|
| `chrome-leak` | `<html>/<head>/<link>` (and top-level `<style>/<script>`) in body_html | error |
| `undefined-token` | `var(--typo)` — a token not in the palette or declared here | error |
| `broken-tabs` | `data-target` with no matching `.tab-panel[data-id]` | error |
| `hardcoded-color` | `#hex` / `rgb()` literals instead of palette tokens | warn |
| `inline-typography` | `font-family` / `font-size` overriding the type stacks | warn |
| `undefined-token` for `--bind-*` | (allowed — created by `data-bind`) | — |
| `nested-card` | `.card` inside `.card` | warn |
| `broken-bind` | `data-bind` with no consumer, or orphan `data-out` | warn |
| `broken-sortable` | `data-sortable` with no `draggable` children | warn |
| `heading-skip` | heading levels that jump (h1 → h3) | warn |
| `img-no-alt` | `<img>` without an `alt` attribute | warn |
Exit code is non-zero when any error-severity rule fires. The output rules
above carry `<!-- rule:ID -->` anchors tying each guidance line to its check,
so the teaching and the enforcement stay in sync. Full-artifact vs body
fragment is auto-detected; force with `--full` / `--fragment`. `--json` emits
machine-readable findings. Contrast ratios are intentionally not checked — the
token pairs are pre-vetted and regex can't judge author-introduced pairs
without false positives.
## Iteration
Edit the spec, re-run `build`, open in a browser. If a layout pattern
repeats across multiple artifacts, that's when a template earns its
keep — otherwise stay in `freeform`.
## Templates: shortcuts for repeat structure
When the **same artifact shape** recurs (status reports week after week, PR
reviews across many PRs, slide decks with consistent navigation), a
template's fixed slot map is worth the translation cost. It enforces
cross-artifact consistency and skips the layout decisions you'd otherwise
re-derive each time.
Use a template only when:
1. You're producing the same artifact shape repeatedly.
2. The repeat structure justifies a fixed slot map.
3. Cross-artifact consistency matters more than per-artifact flexibility.
Otherwise: `freeform`.
```
1. python scripts/build.py list # all templates, one-line summaries
2. python scripts/build.py describe <template> # required keys + JSON skeleton
3. write spec.json # only your content + parameters
4. python scripts/build.py build <template> --spec spec.json --out artifact.html
```
`describe` prints a valid-JSON starter skeleton you can edit in place. For
worked examples, see `references/templates.md` — but only after picking a
template; reading it cold wastes context.
For templates with prose-heavy `*_html` slots (e.g. `summary_html`,
`intro_html`, `details_html`), the same `--set KEY=@FILE` mechanism from the
freeform workflow applies — load the prose from a `.html` file rather than
escaping it into the JSON spec.
There are 21 templates, grouped into 9 categories plus `freeform`:
- `report.*` — status_report, incident_report
- `review.*` — pr_review, code_walkthrough, module_map
- `editor.*` — triage_board, flag_editor, prompt_tuner
- `deck.*` — slide_deck (arrow-key + space navigation)
- `design.*` — design_system, component_variants
- `exploration.*` — comparison_grid, design_directions, implementation_plan
- `research.*` — feature_explainer, concept_explainer
- `diagram.*` — svg_figure_sheet, flowchart
- `prototype.*` — animation_sandbox, click_flow
Some templates with prose-heavy slots take raw HTML in keys ending with
`_html` (e.g. `summary_html`, `intro_html`, `details_html`). Same rules as
`freeform.body_html`: use the inventory above, escape user-supplied content.
## Tests
`tests/test_smoke.py` covers every template with a representative spec plus
explicit security regressions (table escaping, script-tag breakout in
`prompt_tuner`, attribute injection in `flag_editor`, CSS-color injection,
spec mutation in `module_map`). `tests/test_checker.py` covers the `check`
linter — one assertion per rule (fires on the violation, silent on the clean
case). Run with:
```
python composing-html/tests/test_smoke.py # no pytest required
python composing-html/tests/test_checker.py # no pytest required
python -m pytest composing-html/tests -q # if pytest is available
```
When adding or changing a template, add a spec entry and any regression
asserts before merging. When adding a checker rule, add it to both
`scripts/checker.py` and a `<!-- rule:ID -->` anchor in the relevant guidance
line, plus a test assertion.
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