TL;DR
Critical CSS is the minimal set of style rules required to render the above-the-fold content of a page, inlined in a <style> block in the document head so the browser can paint without waiting for external stylesheets.
What is Critical CSS?
Critical CSS is the minimal set of style rules required to render the above-the-fold content of a page, inlined in a <style> block in the document head so the browser can paint without waiting for external stylesheets.
Learn more in our how to improve critical CSS guide, or work with us through our done-for-you speed optimization service.
How Critical CSS is Measured
Lighthouse flags pages where critical CSS could improve rendering time, and tools such as Critters or PurgeCSS can extract it.