Why Do AI Agents Still Write 2019 JavaScript? 'Modern Web Guidance 1.0' Setup Guide

Even when Claude Code, Cursor, or other AI coding agents know a modern CSS or HTML feature by name, they fail 45% of the time when writing code and revert to 2019-era JavaScript workarounds.

Built over 9 months by Google Chrome engineers Paul Irish and Philip Walton—and now stewarded alongside Mozilla Firefox, MDN, and Microsoft Edge in its 1.0 release—GoogleChrome/modern-web-guidance evaluated agents across 200 real-world tasks and 1,700+ Playwright & AST assertions:

• Unguided AI Agent Pass Rate: 55%
• Guided with Modern Web Guidance 1.0: 92%

1. Telemetry-Free Installation (DISABLE_TELEMETRY=1)

By default, the interactive installer sends anonymous usage events to Google Analytics 4. Always prepend DISABLE_TELEMETRY=1:

DISABLE_TELEMETRY=1 npx @google/modern-web-guidance -y

2. Scan Your Existing Codebase (legacy-scan)

Trained on 30,000 GitHub repositories with 92% precision, legacy-scan flags legacy 2019 patterns in your project that can be replaced with native <dialog>, Popover API, field-sizing: content, or CSS.highlights:

npx @google/modern-web-guidance legacy-scan ./src

3. Protect Safari & Firefox Users (~38 Chrome-Only Features)

Roughly 38 of the 217 features in the catalog are currently Chromium-only. Add a .mwgrc file to enforce @supports fallbacks:

{
  "telemetry": false,
  "baseline": "widely-available",
  "requireFallbackForLimitedAvailability": true
}

• 🎛️ Interactive Tool: Legacy 2019 JS vs. Modern Web Primitives Selector
• 📦 GitHub Repository: https://github.com/GoogleChrome/modern-web-guidance