AI Skill
A skill that makes your AI coding assistant check CSS browser support against this site before it answers or writes code.
What it does
The skill points your assistant at cssunlocked.dev/llms.txt, the browser support data this site generates from MDN Browser Compatibility Data. Once installed it kicks in on its own whenever CSS browser support comes up — you never have to name this site.
- Answers support questions
- Reports the version each of Chrome, Edge, Firefox, Safari and Safari (iOS) gained a feature in, and whether a vendor prefix is still needed — read from the data, not from memory.
- Suggests what already works
- When you ask for CSS without naming target browsers, features supported by every target browser are preferred over ones supported by only some.
- Checks versions, not style
- It verifies browser versions only. How the CSS should be written is left to your assistant and to skills that cover implementation patterns.
Download
A zip containing a single SKILL.md file. The data it reads is regenerated whenever MDN Browser Compatibility Data is updated, so the skill itself does not need reinstalling.
Installing
Set it up the way your tool expects. Restart the tool if it does not pick the skill up right away.
- Claude Code
Unzip it and put the
css-unlockeddirectory in one of these.~/.claude/skills/to use it in every projectOfficial documentationcode.claude.com/docs/en/skills.claude/skills/to use it in one project only- Claude apps (desktop and web)
Upload the zip as it is, without unzipping, from Customize → Skills → Add. Turn on “Code execution and file creation” in Settings → Capabilities first.
Official documentationsupport.claude.com/en/articles/12512180- Codex CLI and IDE extension
Unzip it and put the
css-unlockeddirectory in one of these. The Codex built into the ChatGPT desktop app reads the same directories.~/.agents/skills/to use it in every projectOfficial documentationlearn.chatgpt.com/docs/build-skills.agents/skills/in the repository root, to use it in one project only- ChatGPT
Select Plugins → Skills → Create → Upload from your computer in the sidebar, then upload the zip as it is, without unzipping. Skills are available to ChatGPT Business, Enterprise, Healthcare and Edu users.
Official documentationhelp.openai.com/en/articles/20001066-skills-in-chatgpt- Other tools
SKILL.md is plain Markdown with YAML frontmatter. Check the official site of the tool you use.
Trying it out
Once it is installed, ask your assistant something like this.
- Can I use field-sizing: content yet?
- Style this card without a wrapper element.
*The first asks for a fact and gets the versions straight from the data. The second names no browsers, so the assistant reaches for features that already work in all of them.