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Agents Write Yesterday's Web

Coding agents default to the web of old tutorials. Browser makers now hand them current guidance while they work, and every platform owner should do the same.

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Narrated by an AI clone of my voice.

Yesterday's web, today's web Three patterns agents write by habit, each next to what browsers ship today: a modal built from divs becomes the dialog element, a tooltip positioned by a script becomes popover with anchor positioning, and dark mode set by JavaScript becomes color-scheme with light-dark(). WHAT AGENTS WRITE WHAT BROWSERS SHIP A modal built from divs <dialog> A tooltip positioned by a script popover + anchor positioning Dark mode set by JavaScript color-scheme + light-dark()

Researched and drafted with Claude; views are my own.

A coding agent writes the web it was trained on, and that web is mostly old tutorials: dark mode hung on a class or attribute that JavaScript sets, a modal built from divs, a tooltip positioned by a script. Browsers now ship native answers to most of these. The agent has usually seen the modern answer too; it’s just outvoted by ten years of tutorials.

That’s not a quality problem in one project. It’s a distribution problem, and it needs a distribution fix. Teams that write code with agents now ship yesterday’s patterns at scale, and the review that might catch them is increasingly done by another agent with the same habits. Newer models don’t solve it, because volume beats recency, and neither does web search: an agent searches when it’s unsure, and an old pattern doesn’t feel unsure.

Docs have a new reader

Chrome previewed Modern Web Guidance at Google I/O in May, and on 10 November it reached version 1.0, with Firefox and Edge joining Chrome to make it a cross-browser effort. It’s a set of skills a coding agent loads while it works. Each covers a common task, such as a dialog, a form, a tooltip or dark mode, with the modern approach, its pitfalls and its fallbacks, tied to Baseline so the agent knows what is safe for the browsers you support.

Much of the knowledge is on MDN and web.dev already. What’s new is that it’s curated and tested for agents, and delivered to them. Documentation used to wait for a developer to go and read it; now the people who build browsers put their expertise into the agent’s context at the moment the code is written. By Chrome’s own checks, agents equipped with it improved their adherence to modern best practices by 37 percentage points on average in early results, with the caveat that results vary by project, model and prompt. One outside team, note inc., reports pass rates rising from 61% to 94% on its production codebase.

Same knowledge, new delivery Before: browser teams write documentation on MDN and web.dev, which helps only if a developer goes and reads it before writing code. Now: browser teams write skills that are tested and tied to Baseline, and the agent loads them into its context while it writes the code. Same knowledge, new delivery BEFORE Browser teams know what is current Docs pages MDN, web.dev A developer if they go and read it Code often yesterday’s pattern NOW Browser teams know what is current Skills tested, tied to Baseline The agent’s context while it writes Code today’s pattern
The knowledge was always published. What changed is where it lands: in the agent’s context at the moment it writes, instead of on a page a developer might read.

The same move as MCP

This is the argument I’ve been making about airlines, pointed at the web itself. An agent that has to guess its way through a website screenshots it and fumbles the date picker; one that is given tools through MCP or WebMCP books the flight. Tools and knowledge are different things, but the move is the same: the owner hands the agent what it would otherwise have to guess.

The owner hands the agent what it would otherwise guess Two columns. An airline publishes an MCP server whose tools act on its live systems, so the agent books the flight instead of guessing at the website. Browser makers publish skills with knowledge of today's platform, so the agent writes today's web instead of guessing from old tutorials. The owner hands the agent what it would otherwise guess AN AIRLINE The airline owns the booking systems MCP server tools: search, change, book The agent books the flight instead of screenshotting the site BROWSER MAKERS Browser makers own the web platform Skills knowledge: dialog, popover, color-scheme The agent writes today’s web instead of old tutorials tools and knowledge
MCP hands an agent tools and a skill hands it knowledge, but the move is the same: whoever owns the platform publishes what the agent would otherwise have to guess.

That suggests a rule for anyone who owns a platform, an API or a design system: the most frequent reader of your docs is becoming an agent. Ship them in a form an agent loads, not only as a page a person might visit. Concretely, that’s a skill or an MCP server an agent can search and pull from, plus an eval that runs the same task with and without it, the way Chrome tests each guide. For an airline, it’s an MCP server describing what the airline can do. For browsers, it’s a skill describing what the web can do now.

Tried on this site

I ran it on this site this week. The theme toggle, written by a coding agent during the August rebuild, had four quiet bugs: it pinned your choice forever, had no dark mode without JavaScript, never declared both schemes before the stylesheet loaded, and ignored other tabs. One scan flagged the pattern, and the guide behind it showed all four. The fix was written by an agent too, this time with the guide loaded.

Four quiet bugs in this site’s theme toggle Four situations, before and after the guide. Toggling back to your system’s scheme pinned it forever; now it clears the pin and follows the system again. With JavaScript off on a dark system, the site was light; now it is dark from CSS alone. Before the stylesheet loaded, no scheme was declared; now the meta color-scheme tag says light dark. A second open tab kept the old theme; now it follows the toggle. Four quiet bugs in this site’s theme toggle WHENBEFOREAFTER THE GUIDE Toggle back to your system’s scheme pinned it forever clears the pin; follows the system again JavaScript off, dark system light theme dark theme, from CSS alone Before the stylesheet loads no scheme declared meta color-scheme: light dark A second tab open kept the old theme follows the toggle
Each was the common tutorial pattern. One scan flagged it; the guide behind it showed all four.

Nothing dramatic: these are the bugs that ship for years. An agent will look for them now, as long as the people who own the platform tell it what to look for.

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