Web Performance Optimization & Lighthouse · レッスン

フォント読み込みの最適化

`font-display`、プリロード、サブセット化を実装し、レイアウトシフトを起こさずWebフォントを効率的に読み込みます。

「フォント読み込みの最適化」はCoddyKit上の無料Web Performance Optimization & Lighthouseレッスンです。これはレッスン2/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはWeb Performance Optimization & Lighthouse学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Web Performance Optimization & Lighthouseコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

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.

よくある質問

「フォント読み込みの最適化」レッスンは無料ですか？

はい。「フォント読み込みの最適化」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Web Performance Optimization & Lighthouseコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Web Performance Optimization & Lighthouseコースには全4レッスンが含まれています。

「フォント読み込みの最適化」で何を学びますか？

`font-display`、プリロード、サブセット化を実装し、レイアウトシフトを起こさずWebフォントを効率的に読み込みます。ブラウザで直接実行するハンズオンコードでWeb Performance Optimization & Lighthouseを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Web Performance Optimization & Lighthouseを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのWeb Performance Optimization & Lighthouseは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン2/4です。

「フォント読み込みの最適化」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このWeb Performance Optimization & Lighthouseレッスンでコードを書いて実行できますか？

はい。すべてのWeb Performance Optimization & Lighthouseレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

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