HTML Tools

Build single-file applications combining HTML, CSS, and JavaScript that deliver useful functionality without build steps, frameworks, or servers.

Based on Simon Willison's patterns from building 150+ such tools.

When to Use

• "Build me a tool that..."
• "Create a simple app to..."
• "Make a utility for..."
• Client-side data processing or visualization
• Tools that should work offline or be easily shared
• Prototypes that need to ship fast

When to Skip

• Apps requiring authentication or persistent server storage
• Complex state management across many components
• Projects where React/Vue ecosystem benefits outweigh simplicity

Core Principles

1. Single file — Inline CSS and JavaScript. One HTML file = complete tool.
2. No React — Vanilla JS or lightweight libraries only. Explicitly prompt "No React."
3. No build step — Load dependencies from CDNs, not npm.
4. Small codebase — Target a few hundred lines. If larger, reconsider scope.

Development Workflow

Starting a New Tool

1. Begin with Claude Artifacts, ChatGPT Canvas, or similar for rapid prototyping
2. Prompt explicitly: "Build this as a single HTML file. No React. No build steps."
3. For complex tools, use Claude Code or similar coding agents
4. Host on GitHub Pages for permanence

Iterating

1. Remix existing tools as starting points
2. Record LLM transcripts in commit messages for documentation
3. Build debugging tools (clipboard viewer, CORS tester) to understand browser capabilities

State Persistence Patterns

URL State (for shareable links)

Store configuration in URL hash or query params. Users can bookmark or share exact state.

// Save state to URL
function saveState(state) {
  const params = new URLSearchParams(state);
  history.replaceState(null, '', '?' + params.toString());
}

// Load state from URL
function loadState() {
  return Object.fromEntries(new URLSearchParams(location.search));
}

// On page load
const state = loadState();
if (state.color) applyColor(state.color);

localStorage (for secrets and large data)

Store API keys and user preferences without server exposure.

// Store API key securely (never in URL)
localStorage.setItem('openai_api_key', key);

// Retrieve
const key = localStorage.getItem('openai_api_key');

// Prompt user if missing
if (!key) {
  key = prompt('Enter your OpenAI API key:');
  localStorage.setItem('openai_api_key', key);
}

Data Input/Output Patterns

Copy-Paste as Data Transfer

Treat clipboard as the universal data bus. Always include "Copy to clipboard" buttons.

async function copyToClipboard(text) {
  await navigator.clipboard.writeText(text);
  showToast('Copied!');
}

// Rich clipboard (multiple formats)
async function copyRich(html, text) {
  const blob = new Blob([html], { type: 'text/html' });
  const item = new ClipboardItem({
    'text/html': blob,
    'text/plain': new Blob([text], { type: 'text/plain' })
  });
  await navigator.clipboard.write([item]);
}

File Input Without Server

Accept file uploads that process entirely client-side.

<input type="file" id="fileInput" accept=".json,.csv,.txt">

<script>
document.getElementById('fileInput').addEventListener('change', async (e) => {
  const file = e.target.files[0];
  const text = await file.text();
  processFile(text, file.name);
});
</script>

File Download Without Server

Generate and download files entirely in browser.

function downloadFile(content, filename, mimeType = 'text/plain') {
  const blob = new Blob([content], { type: mimeType });
  const url = URL.createObjectURL(blob);
  const a = document.createElement('a');
  a.href = url;
  a.download = filename;
  a.click();
  URL.revokeObjectURL(url);
}

// Usage
downloadFile(JSON.stringify(data, null, 2), 'export.json', 'application/json');

API Integration

CORS-Enabled APIs (call directly from browser)

These APIs allow direct browser requests without a proxy:

LLM API Pattern

Call LLM APIs directly using localStorage-stored keys.

async function callClaude(prompt) {
  const key = localStorage.getItem('anthropic_api_key');
  if (!key) throw new Error('No API key');

  const response = await fetch('https://api.anthropic.com/v1/messages', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
      'x-api-key': key,
      'anthropic-version': '2023-06-01',
      'anthropic-dangerous-direct-browser-access': 'true'
    },
    body: JSON.stringify({
      model: 'claude-sonnet-4-20250514',
      max_tokens: 1024,
      messages: [{ role: 'user', content: prompt }]
    })
  });

  return response.json();
}

GitHub Gists for Persistent State

Use gists as a free, CORS-friendly database.

async function saveToGist(gistId, filename, content, token) {
  await fetch(`https://api.github.com/gists/${gistId}`, {
    method: 'PATCH',
    headers: {
      'Authorization': `token ${token}`,
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({
      files: { [filename]: { content: JSON.stringify(content) } }
    })
  });
}

Advanced Capabilities

Pyodide (Python in Browser)

Run Python, pandas, matplotlib entirely client-side.

<script src="https://cdn.jsdelivr.net/pyodide/v0.24.1/full/pyodide.js"></script>
<script>
async function runPython(code) {
  const pyodide = await loadPyodide();
  await pyodide.loadPackage(['pandas', 'matplotlib']);
  return pyodide.runPython(code);
}
</script>

WebAssembly Libraries

Run compiled libraries in browser:

• Tesseract.js — OCR for images and PDFs
• FFmpeg.wasm — Video/audio processing
• sql.js — SQLite in browser

<!-- Tesseract OCR example -->
<script src="https://cdn.jsdelivr.net/npm/tesseract.js@4/dist/tesseract.min.js"></script>
<script>
async function ocr(imageUrl) {
  const result = await Tesseract.recognize(imageUrl, 'eng');
  return result.data.text;
}
</script>

Output Format

When building an HTML tool, deliver:

HTML TOOL: [name]

[Complete single-file HTML with inline CSS and JavaScript]

FEATURES:
- [Key capability 1]
- [Key capability 2]

USAGE:
[Brief instructions]

HOST: Save as .html and open in browser, or deploy to GitHub Pages.

Reference

Useful patterns for building HTML tools — Simon Willison