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What is Critical CSS and how does it improve web performance?

Updated Apr 28, 2026

Short answer

Critical CSS is the technique of extracting and inlining the CSS required for above-the-fold content to speed up first paint.

Deep explanation

When a browser encounters a <link rel='stylesheet'>, it blocks rendering until the file is downloaded and parsed. By identifying the absolute minimum CSS needed for the viewport on initial load and putting it inline in the <head>, the browser can render immediately. Non-critical CSS is then loaded asynchronously.

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