Next.js 14: Optimize Performance with Deferred CSS Loading

Lazy loading all CSS can indeed lead to a Flash Of Unstyled Content (FOUC). Here's a detailed discussion on why this happens and the role of critical CSS in preventing it:

Flash Of Unstyled Content (FOUC)

FOUC Defined: A Flash Of Unstyled Content (FOUC) occurs when a web page initially displays unstyled content before the CSS is fully loaded and applied. This can be a jarring experience for users and can negatively impact the perceived performance and aesthetics of a website.

Causes of FOUC:

1. Delayed CSS Loading: If the CSS is loaded lazily, it means the browser has to wait for the CSS to be fetched and applied after the initial HTML content is rendered. During this delay, the content is displayed without styles.
2. Network Latency: Any delays in fetching CSS files due to network latency will contribute to the duration of FOUC.
3. Rendering Order: The browser renders HTML content as it is parsed. If the CSS is not available during this parsing process, the HTML content will be displayed in its unstyled form.

Preventing FOUC with Critical CSS

Critical CSS Defined: Critical CSS refers to the CSS rules required to style the above-the-fold content of a webpage. This is the portion of the webpage that is visible to the user before any scrolling.

Benefits of Critical CSS:

1. Improved Perceived Performance: By ensuring the critical CSS is loaded and applied immediately, the content that the user sees first is styled correctly, leading to a better initial impression.
2. Reduced FOUC: Loading critical CSS inline or as a high-priority resource ensures that the above-the-fold content is styled as soon as it is rendered by the browser, minimizing or eliminating FOUC.

Implementation Strategies:

Inline Critical CSS: Embedding the critical CSS directly in the HTML document ensures that it is available immediately as the browser parses the HTML. This approach eliminates any delay in applying styles to above-the-fold content.

<style>
/* Critical CSS here */
body {
    margin: 0;
    font-family: Arial, sans-serif;
}
.header {
    background-color: #f8f9fa;
    padding: 10px;
    text-align: center;
}
</style>
        

Asynchronous Loading of Non-Critical CSS: The rest of the CSS, which is not immediately needed for rendering above-the-fold content, can be loaded asynchronously. This can be done using rel="preload" or dynamically loading the CSS file via JavaScript.

<link rel="preload" href="styles.css" as="style" onload="this.onload=null;this.rel='stylesheet'">
<noscript><link rel="stylesheet" href="styles.css"></noscript>
        

Using Tools to Extract Critical CSS: Tools like Critical, Penthouse, or online services can be used to automatically extract the critical CSS from your full CSS. These tools analyze your webpage and generate the minimal CSS required to style the above-the-fold content.

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Lazy loading all CSS can indeed cause a Flash Of Unstyled Content (FOUC) as the browser displays the unstyled content until the CSS is fully loaded and applied. To prevent this, it is essential to load critical CSS above the fold. By embedding critical CSS directly in the HTML and loading non-critical CSS asynchronously, you can ensure a smoother and visually consistent user experience, improving both perceived and actual performance.