Chrome's Modern Web Guidance is a set of agent skills that embeds current browser API knowledge and best practices into AI coding workflows. Without it, AI agents often default to legacy patterns — adding JavaScript libraries for tooltips, custom modal timers, or synchronous filtering — because their training data lags behind the evolving web platform. A side-by-side comparison of two Next.js task manager apps (one built with a standard agent, one with Modern Web Guidance enabled) shows concrete differences: the guided agent used native `<dialog>` with CSS entry/exit animations instead of a `setTimeout`-based portal, CSS Container Queries instead of media queries, CSS Anchor Positioning instead of Floating UI, and `scheduler.yield()` to keep search filtering non-blocking. Installation is via `npx modern-web-guidance@latest install` or agent-specific commands for Gemini CLI, Claude Code, Copilot CLI, and others. Declaring a Baseline browser target in the agent instruction file (e.g., `AGENTS.md`) lets the guidance calibrate when native APIs are safe versus when fallbacks are needed. The guidance improves the agent's starting point but does not replace code review, accessibility testing, or browser compatibility validation.
