css-sota-mcp

An MCP server that answers what CSS you can actually ship today — from live Baseline data and MDN browser-compat-data, not from a model's training set.

Agents are confidently wrong about browser support. They will tell you anchor-name is fine, or that :has() needs a polyfill, depending on when their weights were frozen. This server replaces the guess with the current answer.

• Endpoint — https://css-sota-mcp.lusrodri.workers.dev/mcp (Streamable HTTP, no auth)
• Docs — css-sota-mcp.pages.dev
• Registry — io.github.LuSrodri/css-sota-mcp, listed in the official MCP Registry

Tools

check_support and audit_css answer with no network call at all — the data they need is compiled into the Worker.

dont_make_me_think

Named after Steve Krug's rule: a page should be self-evident. Two modes.

mode: "guidelines" returns the principles to design against — Nielsen's 10 heuristics, Hick's and Fitts's laws, WCAG 2.2, neurodiversity-inclusive design, motion and microinteractions (including when Lottie or Rive earn their bundle cost), SVG craft and animation, light-first theming, lightness, responsiveness. Filter with topic. The knowledge base is mcp/src/data/ux-guidelines.json; every principle carries its rationale, actionable rules and a source.

mode: "review" checks HTML and CSS — or a fetched url — and reports what violates which principle, with the line and the evidence.

It reads source; it does not render it. A Worker has no layout engine, so the review cannot measure computed contrast, real target sizes, or where focus actually lands. It catches what is visible in the markup: missing alt, blocked zoom, animation with no reduced-motion path, a removed focus ring, a dark-only palette, vague link text, a nav past Hick's range. A clean result is a floor, not a pass, and the tool says so in its own output.

audit_css targets

Two target styles, because they answer different questions:

• A Baseline level — baseline-widely, baseline-newly. Asks "is this interoperable enough to ship?", judged against web-features' Baseline status.
• An explicit browser list — chrome 120, safari 17.4, firefox 128. Asks "does this work for my users?", judged against per-browser versions.

Browserslist queries (last 2 versions, >0.5%) are not accepted. Resolving them needs usage data this server does not carry, and approximating them would produce confidently wrong audits — exactly the failure mode the server exists to fix. The tool says so rather than guessing.

Connect

claude mcp add --scope user --transport http css-sota https://css-sota-mcp.lusrodri.workers.dev/mcp

--scope user registers it once for every project on the machine. Leave it out and the server is added to the current project only.

https://css-sota-mcp.lusrodri.workers.dev/mcp