Frontend & browser·Loading, assets and the critical path
the styles for the first screen are inlined in the head and the rest of the stylesheet loads afterwards.
Critical CSS
Also calledabove-the-fold CSS, inlined critical styles
Extracting the rules needed to render the initial viewport and inlining them, so first paint does not wait on a stylesheet request. It reliably improves first contentful paint on a slow connection. It is also fragile: the extraction has to be regenerated with every change, the inlined bytes are not cacheable, and getting the set wrong produces a flash of unstyled content instead.