What is critical CSS?

Critical CSS is the small subset of styles needed to draw what the visitor sees first, inlined into the page so it renders without waiting:

• The rest loads afterwards - the full stylesheet arrives without holding up the first paint.
• It targets the blank-screen problem - a stylesheet is render-blocking by nature.
• Most platforms automate it - a plugin or your framework usually generates it.

Worth doing once the obvious wins - images, unused scripts, caching - are already done.