Composing HTML Skill for Claude

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.

46k tokens

context cost

the whole folder, loaded on every use

files

ships runnable scripts

copies elsewhere

how many repositories repackaged it

137

stars on the repo

on the repository, not the skill itself

Install

one command, takes just this skill from the repository

npx skills add https://github.com/oaustegard/claude-skills --skill composing-html

The instruction itself

19 sections, as written by the author

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

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

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 [email protected] \
       --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 [email protected]

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 [email protected] (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

<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

<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

<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:

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.

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.

body_html is HTML, not a JSON dialect. Write <section>, <h2>,

<ul class="bullets"> directly. No translation layer.

Anything in an _html field is inserted verbatim — escape any

user-supplied content yourself. All other string values are

HTML-escaped automatically.

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  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:

You're producing the same artifact shape repeatedly.
The repeat structure justifies a fixed slot map.
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  anchor in the relevant guidance

line, plus a test assertion.

Other skills for the same job

different authors, same section of the catalogue

Theme Factory

by anthropics

vendor ×16

Toolkit for styling artifacts with a theme. These artifacts can be slides, docs, reportings, HTML landing pages, etc. There are 10 pre-set themes with colors/fonts that you can apply to any artifact that has been creating, or can generate a new theme on-the-fly.

Applies Anthropic's official brand colors and typography to any sort of artifact that may benefit from having Anthropic's look-and-feel. Use it when brand colors or style guidelines, visual formatting, or company design standards apply.

Suite of tools for creating elaborate, multi-component claude.ai HTML artifacts using modern frontend web technologies (React, Tailwind CSS, shadcn/ui). Use for complex artifacts requiring state management, routing, or shadcn/ui components - not for simple single-file HTML/JSX artifacts.

11k tokens scripts

Web Artifacts Builder

Create distinctive, production-grade frontend interfaces with high design quality. Use this skill when the user asks to build web components, pages, or applications. Generates creative, polished code that avoids generic AI aesthetics.

Guidance for distinctive, intentional visual design when building new UI or reshaping an existing one. Helps with aesthetic direction, typography, and making choices that don't read as templated defaults.

5k tokens