The library you’re thinking of is Wikimedia Foundation’s CSS Sanitizer, often referred to as:

It’s a PHP library used in MediaWiki to safely filter and whitelist CSS, preventing malicious styles (e.g. position:fixed, behavior, expression, etc.) from being injected.

In the MediaWiki codebase, it’s also tied to the broader Sanitizer system, but the standalone reusable piece is:

👉 In short: it’s not general CSS handling—it’s a security-focused CSS whitelist engine.