Critical CSS improves webpage load speed by prioritizing essential styles for above-the-fold content. 1) It identifies minimal required styles for initial rendering. 2) Tools like Penthouse, Critical, and Lighthouse help extract critical styles. 3) Styles are inlined in HTML to reduce render-blocking. 4) Non-critical CSS is deferred via async loading or media attributes. 5) Maintenance is crucial to avoid FOUC and flickering. 6) Frameworks like Next.js and Gatsby offer built-in support. Proper implementation and regular updates ensure fast, consistent rendering without layout issues.
Getting the CSS delivered efficiently is a big part of making web pages load fast, especially above-the-fold content. One solid way to do that is with Critical CSS techniques. The idea is simple: load only what’s needed first, and defer the rest.

What is Critical CSS?
Critical CSS (sometimes called “above-the-fold” CSS) is the minimal set of styles needed to render the visible part of a webpage right when it loads. Everything else — like styles for sections users can’t see until they scroll — can be loaded later.

The main benefit? Faster perceived load times. Without Critical CSS, browsers have to download and parse all CSS before rendering the page, which causes delays. By inlining only the essential styles directly into the HTML, you let the browser paint the screen much sooner.
How to Extract and Apply Critical CSS
There are a few tools out there that help extract Critical CSS automatically. Some popular ones include:

- Penthouse – works well for static sites
- Critical – a PostCSS plugin that integrates into build processes
- Google’s CriticalCSS tool
- Lighthouse – gives hints but doesn’t extract for you
The general process looks like this:
- Identify key HTML templates or routes
- Use a headless browser to capture used styles on initial load
- Inline those styles in a
<style></style>
tag inside the - Defer loading the full stylesheet or load it asynchronously
This isn’t something you can just set once and forget — if your layout changes, your Critical CSS might need updating too.
Managing Non-Critical CSS
Once the critical part is handled, the rest of your CSS should be loaded in a way that doesn’t block rendering. Here are some common approaches:
- Load full CSS asynchronously using JavaScript
- Use
rel="preload"
withonload
to load styles after initial render - Add media attributes to non-critical stylesheets so they don’t block rendering
For example, you can use a script like this to load the full CSS after the critical part has been applied:
<link rel="stylesheet" href="styles.css" media="print" onload="this.media='all'">
This tells the browser not to block rendering while downloading the file, but still apply it once it's ready.
Watch Out for Maintenance and Flicker
One thing people often overlook is how hard it is to maintain Critical CSS over time. If your site layout or design system changes frequently, your Critical CSS could become outdated without you realizing it. That leads to broken layouts or flash of unstyled content (FOUC).
Also, improper implementation can cause visual flickering — like when the critical styles are slightly off from what’s actually needed. To reduce this risk:
- Test different viewport sizes
- Use real user data or screenshots to validate extraction
- Don’t skimp on testing after every major layout change
It’s also worth noting that frameworks like Next.js and Gatsby have built-in support or plugins for handling Critical CSS, so you don’t always have to roll your own solution.
That’s pretty much how you handle CSS delivery with Critical CSS. It helps pages feel faster without sacrificing style structure — as long as you keep things updated and test thoroughly.
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