youmightnotneed

Is it CSS yet?

Find the CSS, HTML, or Web API that replaces your JavaScript dependencies. One rule catalog, read by a website, a CLI, an MCP server and an agent skill.

youmightnotneed.dev

Start here

npx youmightnotneed                             # the nearest package.json
npx youmightnotneed --package swiper --verbose  # one package, with conditions
npx youmightnotneed --since 2026-03-01          # only what crossed since then

  Up to 89.0 kB across 10 dependencies

+ Baseline widely available · safe to use today

  Locale-aware date and time formatting (20.3 kB)
    you have  moment
    native    Intl.DateTimeFormat
    keep it if 5 conditions apply, see --verbose

Or paste a package.json at the site. Nothing is stored: the report is encoded in the URL.

For an agent, whichever the host supports:

npx youmightnotneed-mcp                         # MCP server, over stdio
npx skills add jomaendle/youmightnotneed        # the skill
curl -sS https://youmightnotneed.dev/llms.txt   # or just read it

/plugin marketplace add jomaendle/youmightnotneed installs the skill and registers the MCP server in one step.

What a finding claims

A dependency in package.json is not proof of what it is used for. Someone installs Framer Motion for layout animations, not for fade-ins. So a finding is a conditional:

If you're using swiper for a horizontal gallery with prev/next and dots, CSS scroll-snap with ::scroll-button() and ::scroll-marker() covers that case.

Every rule carries the conditions where the library is still the right call. A rule with an empty unless list fails the schema, so it cannot be added by accident. Sizes are "up to", because they assume a full replacement that may not apply to you.

What this is not

Three questions sit next to this one and are answered better elsewhere.

What is left is the part no linter covers: multi-line behaviour someone wrote by hand. A focus trap, a scroll lock, a carousel built from scroll listeners. Nothing is installed for any of it, so no package.json scan finds it either. Those shapes are on every rule that has them, and the agent skill reads them.

Where the numbers come from

No browser version and no support tier in this repo is written by hand.

flowchart LR
  WF["web-features"] --> R["refresh scripts"]
  BCD["browser-compat-data"] --> R
  R --> S["committed snapshot"]
  S --> W["website"]
  S --> C["CLI"]
  S --> M["MCP"]
  S --> K["skill"]

A rule stores web-features IDs and a build step turns them into widely, newly or limited, so every change shows up in a diff. A rule is only as available as its weakest required feature. Prose works the same way: a rule writes {{safari:api.Crypto.randomUUID}} and pnpm refresh:support resolves it, so a token no source confirms fails the refresh and a typed version fails the tests.

A rule may also point at Google Chrome's modern-web-guidance (Apache-2.0) by ID for the implementation, and a guide that disappears upstream fails the freshness check rather than shipping as a dead link.

Layout

packages/catalog   @jomae/catalog, the rules and detect()
packages/cli       npx youmightnotneed
packages/mcp       npx youmightnotneed-mcp
apps/web           youmightnotneed.dev
skills             the agent skill, with a generated catalog reference
scripts            snapshot generators and the freshness check

detect() is pure: a parsed dependency map in, findings out. No filesystem, no network, no clock. Every surface calls the same one, which is why they cannot disagree, and a test asserts it by reading the source.

Contributing

pnpm install
pnpm verify      # lint, typecheck, tests, freshness, copy
pnpm dev         # the website
pnpm refresh     # re-snapshot Baseline data, sizes, guides and the skill

A rule is one file in packages/catalog/src/rules/, exported from the index. The schema says what is missing; pnpm verify says the rest. Write unless first, because an answer that always says "the platform covers it" is worse than no answer.

.claude/skills/finding-rules/SKILL.md covers deciding there is a rule to write. .claude/skills/adding-a-rule/SKILL.md covers writing it. Any change to a published package wants a changeset: run pnpm changeset.

Licence

MIT. The catalog is the useful part, so take it.

{
  "$schema": "https://json.schemastore.org/claude-code-marketplace.json",
  "name": "jomaendle",
  "description": "Johannes Mändle's Claude Code plugins",
  "owner": {
    "name": "Johannes Mändle",
    "url": "https://www.jomaendle.com"
  },
  "plugins": [
    {
      "name": "youmightnotneed",
      "source": "./",
      "description": "Check whether a JavaScript dependency has a native CSS, HTML or Web API equivalent",
      "version": "0.2.0"
    }
  ]
}