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Web Performance Optimization & Lighthouse · Lesson

Optimizing Font Loading

Implement `font-display`, preloading, and subsetting to ensure web fonts load efficiently without causing layout shifts.

Optimizing Font Loading is a free Web Performance Optimization & Lighthouse lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Web Performance Optimization & Lighthouse learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Why Optimize Fonts?

Web fonts make websites beautiful and unique, but they can also be a major source of performance bottlenecks. Large font files mean longer download times, which directly impacts your site's overall speed and user experience.

Slow font loading can lead to issues like 'Flash of Unstyled Text' (FOUT) or 'Flash of Invisible Text' (FOIT). In this lesson, we'll learn practical techniques to load fonts efficiently.

Control Font Loading with `font-display`

The font-display CSS property gives you control over how web fonts load and display. It tells the browser what strategy to use while a font is still downloading, helping to prevent jarring visual shifts.

  • auto: The browser's default behavior.
  • block: Short block period (invisible text), then infinite swap.
  • swap: Very short block period (invisible text), then infinite swap.
  • fallback: Very short block, then short swap, then permanent fallback.
  • optional: Very short block, no swap, permanent fallback.

`font-display: swap` in Action

font-display: swap is a popular choice for balancing performance and aesthetics. It tells the browser to use a fallback font immediately. Once your custom web font is downloaded, it 'swaps' in, replacing the fallback.

This ensures text is always visible, prioritizing content readability over font styling during the initial load.

@font-face {
  font-family: 'MyWebFont';
  src: url('mywebfont.woff2') format('woff2');
  font-display: swap;
}

`optional` and `fallback` Strategies

Other font-display values offer different trade-offs:

  • fallback: Gives the custom font a very short block period (around 100ms) and a short swap period (around 3s). If the custom font isn't ready within these times, it uses a fallback font permanently.
  • optional: Similar to fallback, but the browser decides if the custom font is downloaded at all, based on network conditions. If the font is not downloaded quickly, the fallback is used permanently, avoiding any layout shift.

Preloading Critical Fonts

Normally, browsers discover fonts when they parse your CSS. However, you can tell the browser to fetch a resource *earlier* than it would normally discover it using preloading.

For fonts, <link rel="preload"> means the browser can start downloading them even before the CSS that uses them is fully parsed. This significantly reduces the time until text is displayed with the correct font.

How to Preload a Font

You can preload fonts by adding a <link> tag in your HTML's <head> section. It's crucial to include as="font" and crossorigin to ensure the font is fetched correctly and to prevent double downloads.

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Preload Font Example</title>
  <link rel="preload" href="mywebfont.woff2" as="font" type="font/woff2" crossorigin>
  <style>
    @font-face {
      font-family: 'MyWebFont';
      src: url('mywebfont.woff2') format('woff2');
      font-display: swap;
    }
    body {
      font-family: 'MyWebFont', sans-serif;
    }
  </style>
</head>
<body>
  <h1>Hello CoddyKit!</h1>
  <p>This text uses a preloaded web font.</p>
</body>
</html>

Reducing Font Size with Subsetting

Font files often contain hundreds, or even thousands, of characters. Many of these, like special symbols or characters for different languages, might not be used on your specific website.

Font subsetting is the process of creating a smaller version of a font file that only includes the specific characters (glyphs) your website actually needs. This drastically reduces the file size.

Why Subset Your Fonts?

Subsetting has a direct and significant impact on your website's performance:

  • Faster Downloads: Smaller font files download much quicker, leading to improved page load times.
  • Reduced Bandwidth: This saves data for your users, which is especially important for those on mobile plans.
  • Better User Experience: Less waiting means users see your content, styled correctly, much faster.

Tools like Font Squirrel's Webfont Generator or Google Fonts can help you subset.

Test Your Font Knowledge

Which of the following are effective strategies for optimizing web font loading and performance?

Font Loading Recap

We've covered essential techniques for optimizing web font loading:

  • font-display: This CSS property controls how fonts render during download, preventing FOUT and FOIT.
  • Preloading: Using <link rel="preload"> in your HTML fetches critical fonts earlier, speeding up their availability.
  • Subsetting: Reduces font file size by including only the characters your website actually needs.

By applying these strategies, you can significantly improve your website's perceived performance and deliver a smoother user experience.

Frequently asked questions

Is the “Optimizing Font Loading” lesson free?

Yes — the full text of “Optimizing Font Loading” is free to read here on the web, and the Web Performance Optimization & Lighthouse course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Web Performance Optimization & Lighthouse course, upgrade to CoddyKit PRO.

What will I learn in “Optimizing Font Loading”?

Implement `font-display`, preloading, and subsetting to ensure web fonts load efficiently without causing layout shifts. You practise Web Performance Optimization & Lighthouse with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Web Performance Optimization & Lighthouse?

No prior experience is required. Web Performance Optimization & Lighthouse on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Optimizing Font Loading” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Web Performance Optimization & Lighthouse lesson?

Yes. Every Web Performance Optimization & Lighthouse lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Critical CSS and CSS Delivery
  2. Optimizing Font Loading
  3. CSS-in-JS Performance Impact
  4. Reducing Unused CSS
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