TL;DR
NitroPack’s Generate Critical CSS feature extracts the CSS needed to style above-the-fold content and loads it early, so your site appears faster to visitors. This improves metrics like First Contentful Paint (FCP) and Largest Contentful Paint (LCP), making your site feel instantly responsive.
When someone visits your site, how quickly they see meaningful content can make or break their experience.
If the page feels slow or blank for too long, users bounce. One of the biggest culprits?
Render-blocking CSS, especially styles that aren’t needed until much later in the page.
That’s where Critical CSS comes in. Read on to learn how you to use this optimization technique to speed up your WordPress site.
What Does “Critical CSS” Mean?
Critical CSS refers to the specific CSS rules required to render the content above the fold—the part of the page that’s immediately visible without scrolling.
By isolating and loading only these essential styles first, you allow the browser to paint the visible portion of your site faster, while deferring non-critical styles to load afterward.
Without Critical CSS set in place, the browser:
- Downloads the full CSS file (which might be huge)
- Waits to parse and apply it
- Then starts rendering the page
With Critical CSS, the browser:
- Immediately applies the most important styles
- Renders the above-the-fold content sooner
- Loads the rest of the styles in the background
Test NitroPack yourself
With vs Without Critical CSS (Example)
Without Critical CSS, your site’s stylesheets load all at once—even if only a small portion is needed to render what users first see. This delays key metrics like FCP and LCP, and creates a longer blank-screen experience.
With Critical CSS, only the essential styles are loaded upfront, so above-the-fold content appears immediately. The result? A snappier, more engaging experience from the very first second.
| Scenario | Without Critical CSS | With Critical CSS |
| FCP (First Contentful Paint) | Delayed—browser waits for full CSS | Faster—above-the-fold content loads immediately |
| LCP (Largest Contentful Paint) | Slower for hero images/headlines | Improved—important content styled early |
| Render-blocking | Entire CSS file is blocking | Only essential styles load first |
| User perception | Blank screen for longer | Page appears instantly usable |
Why Is Generating Critical CSS Important?
Modern websites load dozens of stylesheets—from themes, plugins, page builders, and more. But the truth is that most of this CSS isn’t needed right away.
Generating and applying Critical CSS solves this by:
- Reducing render-blocking time – Only necessary styles are loaded early
- Improving performance metrics – Especially FCP and LCP, which affect SEO and UX
- Making your site feel faster – Even before everything loads, users see something meaningful
- Increasing user retention – Faster visual response equals lower bounce rates
And while the benefits are clear, manually generating Critical CSS for every page is time-consuming and error-prone.
That’s why NitroPack automates the process, so you get all the performance gains without the hassle.
Bonus: Helps Resolve PageSpeed Insights Warnings
Pages without Critical CSS often trigger PSI and Lighthouse warnings like:
- Reduce unused CSS
- Generate Critical CSS
- Eliminate Render-Blocking Resources
- Avoid Excessive DOM Size
By generating and inlining Critical CSS, NitroPack fixes all these warnings and also boosts your performance metrics, especially on mobile, where even small delays are more noticeable.
NitroPack’s Generate Critical CSS Explained
NitroPack automates the entire Critical CSS process for WordPress sites—no need to extract styles manually, write custom rules, or worry about page layouts breaking.
Here’s how it works:
- Scans each page individually to identify above-the-fold content
- Extracts only the CSS needed to render that visible section
- Inlines this Critical CSS into the page’s
<head>for instant loading - Defers loading of the full CSS bundle until after initial paint
- Updates dynamically when content or layout changes
This ensures the page renders quickly and consistently across desktop and mobile, homepages and inner pages.
NitroPack also handles layout shifts by:
- Respecting font sizes, image dimensions, and spacing rules
- Coordinating with other optimizations like Lazy Load and Preload
NitroPack vs Manually Generating Critical CSS
Manually generating Critical CSS is time-consuming, fragile, and hard to scale. It often requires custom extraction, coding expertise, and ongoing maintenance—especially when layouts change or new pages are added.
NitroPack takes care of all of this automatically. It generates page-specific Critical CSS, adapts to changes instantly, and ensures your site is optimized for both desktop and mobile—without writing a single line of code.
| Feature | Generating Critical CSS Manually | Generating Critical CSS w/ NitroPack |
| Setup | Requires custom extraction and code changes | Fully automated |
| Page-specific rules | Must be handled individually | Automatically generated per page |
| Maintenance | Breaks easily when layout changes | Dynamic updates handled for you |
| Mobile support | Requires separate stylesheets | Optimized for all devices |
How to Enable Generate Critical CSS in NitroPack
Getting started is simple:
- Log into your NitroPack dashboard
- Go to Cache Settings
- Navigate to the HTML & CSS menu
- Toggle on Generate Critical CSS
- Save your changes
That’s it—NitroPack will automatically begin generating Critical CSS for every page.
FAQs
Will Critical CSS break my layout?
No. NitroPack extracts only the safe-to-load styles and continuously monitors for layout shifts.
Does it work on every page?
Yes—Critical CSS is generated per URL, ensuring accuracy and consistency.
What happens if my layout changes?
NitroPack re-evaluates the page and regenerates Critical CSS automatically.
Can I manually customize Critical CSS?
Advanced users can override or adjust Critical CSS using custom settings, though it’s rarely needed.
