tl;dr: Modern Web Guidance is a CLI-based skill (stewarded by Chrome, Edge, Firefox, and MDN teams) that injects curated, token-efficient guidance on ~250 modern web platform features into coding agents' context windows, steering them away from legacy JS patterns toward native browser APIs with responsible fallbacks and Baseline-aware compatibility data. It works via local semantic search (offline, no API keys) and installs across Claude Code, Codex, Copilot, Antigravity, and others. Published evals show significant uplift—e.g., Claude Opus jumping from 57% to 88% task pass rate, and Gemini from 58% to 94%.