Critical CSS is a technique that extracts and inlines the minimal CSS required to render above‑the‑fold
content. By providing those essential styles upfront, the browser can paint the initial view without
waiting for external style sheets to download and parse. This improves perceived performance and can
positively influence FCP and LCP. WP Lightning Load’s Critical CSS module lets you enable this behavior
globally, optionally separate mobile rules (useful for themes with very different mobile layouts), and
schedule periodic regeneration to keep up with design changes.
You should consider enabling Critical CSS when your pages rely on large or numerous style sheets, or when
render‑blocking CSS is a top offender in performance audits. Start with “above‑the‑fold only” and
validate that key elements—headers, navigation, hero content—render correctly without flashes of
unstyled content. If parts of the fold still appear unstyled initially, expand the critical set or
ensure that essential font declarations, layout utilities, and base components are included. For highly
dynamic or personalized pages, be conservative to avoid embedding rules that don’t apply consistently.
Critical CSS pairs well with deferring non‑critical scripts and lazy loading media. As with all
optimizations, test on real devices and networks, and confirm that visual stability remains good. With
careful configuration, Critical CSS can provide a meaningful boost without adding significant
maintenance overhead.