Why it matters
Baseline is the shared answer to "can I use this yet?". The web-features project, run by the W3C WebDX group with browser vendors, tracks when each web platform feature became available in the current versions of Chrome, Edge, Firefox and Safari. A feature is newly available once it ships in all of them, and widely available 30 months later. That gives a single date to check against instead of a grid of version numbers.
This checker parses your CSS and maps what it finds to web-features: properties and their values, selectors like :has() and nesting, at-rules like @container and @layer, and media features. Each one gets its Baseline status and the date it reached it, and your target (widely available, a Baseline year, or a date) decides which pass. Features used inside @supports are marked as guarded, since you've already planned a fallback. The data is a dated snapshot bundled with the page, so the result doesn't change under you; the date is shown with every report.
How to use it
- Paste your CSS.
- Pick a Baseline target: widely available, a year, or a date.
- Check the table: anything that misses your target is listed first.
- Copy the Markdown report into your pull request.
FAQ
What's the difference between newly and widely available?
Newly available means the feature works in the latest version of every core browser. Widely available means it's been that way for 30 months, so most people's browsers support it.
Where does the data come from?
From the web-features package that powers Baseline on MDN and caniuse. The snapshot date is shown on the page.
Does it check JavaScript or HTML?
No, only CSS. It recognises CSS features, not script APIs or HTML elements.
Is my CSS uploaded?
No. It's checked in your browser against the bundled data.
Credits and licences
Browser support data from web-features (Apache License 2.0), W3C WebDX Community Group.
