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Should CSS always precede JavaScript?

It’s generally considered best practice to link or load CSS before JavaScript—usually placing CSS in the <head> and JavaScript at the end of the <body> or using defer. This ensures the browser can render styles sooner and avoid blocking page rendering. However, it’s not an absolute rule; there are exceptions based on specific performance or loading strategy needs.

1. Typical Loading Order

  1. CSS in the <head>

    • The browser downloads and applies styles before rendering the page layout.
    • Prevents the “flash of unstyled content” (FOUT) or layout shift issues.

  2. JavaScript at the end of <body> or with defer

    • If scripts are loaded synchronously in the <head>, they block the page from rendering until they finish.
    • Placing scripts at the bottom of <body> (or using async/defer) allows the page to render first, improving perceived performance.

Example

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Example</title>
  <!-- CSS link in head -->
  <link rel="stylesheet" href="styles.css">
</head>
<body>

  <h1>Hello, World!</h1>

  <!-- JavaScript at the bottom -->
  <script src="script.js" defer></script>
</body>
</html>

2. Why Load CSS First?

  1. Preventing Rendering Delays

    • Browsers typically won’t render a page until they’ve downloaded and parsed linked CSS (to avoid sudden style changes).
    • Having CSS in the head ensures minimal blocking time for the critical rendering path.

  2. User Experience

    • CSS defines how content looks—loading it early helps avoid content popping in unstyled, then jumping around once styles apply.

  3. Performance

    • Inlining or loading critical CSS first can reduce layout shifts.
    • Non-critical or large CSS might be deferred or loaded asynchronously to optimize performance, but the main idea remains: the initial, critical styles get priority.

3. Exceptions or Special Cases

  1. Non-Critical CSS

    • Sometimes, you might load certain styles (e.g., below-the-fold or theme variations) later or asynchronously using media attributes or a JS-based approach for performance reasons.

  2. JavaScript That Manipulates Styles or Requires Early Execution

    • If a script modifies the DOM before the user sees it (e.g., removing certain nodes, adjusting layout logic), you might place it in the <head>—but be cautious about blocking.

  3. Inlining Critical CSS

    • Another advanced strategy is placing critical CSS inline in the <head> for immediate rendering, and loading the rest of the styles asynchronously. This can be beneficial in performance-sensitive scenarios.

4. Impact on SEO and Accessibility

  • SEO: Search engines index the rendered content. Quick, accurate rendering (with styles applied) helps reduce bounce rates and improves user experience.
  • Accessibility: Ensuring your CSS is promptly loaded helps screen readers and other assistive technologies display content in a stable layout from the start.

5. Summary

  • While not a hard rule, it’s best practice to let browsers load CSS before JavaScript, typically by placing CSS in the <head> and scripts at the end of <body> or using defer.
  • This approach optimizes rendering, prevents blocking, and improves the perceived load time.
  • If you have special needs (e.g., immediate DOM manipulation, conditional loading), you might adjust the order—just ensure it doesn’t degrade performance or user experience.

Ultimately, loading CSS first usually yields faster, more stable page rendering, aligning with standard performance guidelines.

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