Lazy Loading Fonts

Lazy loading is a design pattern in web development where resources (such as images, scripts, or fonts) are loaded only when they are needed rather than when the page initially loads. This strategy helps optimize the loading time and rendering speed of a webpage, as it reduces the amount of data fetched and processed upfront.

In the context of fonts, lazy loading ensures that fonts not immediately visible to the user are only loaded when required, reducing the time-to-first-paint (TTFP) and improving the perceived performance of a web page.

How Does Lazy Loading Work for Fonts?

1. Load Critical Fonts First: Load only the fonts that are required for rendering the content visible in the initial viewport. This ensures that users see styled content quickly.

2. Defer Non-Essential Fonts: Fonts used in offscreen or less critical elements (e.g., footer or secondary sections) are loaded later, usually after the page's primary content is visible.

3. JavaScript-Based Lazy Loading: JavaScript libraries or custom scripts can monitor when certain elements come into view (using techniques like the Intersection Observer API) and then trigger the loading of additional fonts.

Examples of Lazy Loading

1. Defer Loading with JavaScript:

   • Use a script to load fonts after the main content is rendered:

     <script>
       window.addEventListener('load', () => {
         const font = new FontFace('MyFont', 'url(/path-to-font.woff2)');
         font.load().then(() => {
           document.fonts.add(font);
           document.body.style.fontFamily = 'MyFont, sans-serif';
         });
       });
     </script>
     

2. Subset Fonts for Critical Sections:

   • Serve a minimal font subset for the initial viewport and lazy load the full font for the rest of the page:

     @font-face {
       font-family: 'MyFont';
       src: url('/fonts/myfont-basic.woff2') format('woff2');
       unicode-range: U+0020-007F; /* Basic Latin */
     }
     

3. Intersection Observer API:

   • Load fonts dynamically when an element containing text comes into view:

     const observer = new IntersectionObserver((entries, observer) => {
       entries.forEach(entry => {
         if (entry.isIntersecting) {
           const font = new FontFace('LazyFont', 'url(/path-to-font.woff2)');
           font.load().then(() => {
             document.fonts.add(font);
           });
           observer.unobserve(entry.target);
         }
       });
     });
     
     observer.observe(document.querySelector('.lazy-font-section'));
     

Advantages of Lazy Loading Fonts

1. Faster Initial Page Load: Only essential resources are loaded initially, speeding up the time it takes for the page to appear usable.

2. Reduced Bandwidth Usage: Saves data for users, especially important for mobile and low-bandwidth connections.

3. Improved Performance Metrics: Metrics like First Contentful Paint (FCP) and Largest Contentful Paint (LCP) improve because the browser prioritizes critical content.

4. Better User Experience: Users see content faster, with less waiting for the page to load fully.

Potential Drawbacks of Lazy Loading Fonts

1. Flash of Unstyled Text (FOUT): If fonts take too long to load, the user might see text rendered in a fallback font before the lazy-loaded font appears.

2. Increased Complexity: Lazy loading adds complexity to your codebase and may require careful testing to ensure it doesn't interfere with functionality.

3. Delays in Displaying Non-Critical Content: If users scroll quickly to sections with lazy-loaded fonts, there may be a visible delay while the font is fetched and rendered.

Lazy loading is a powerful tool when used correctly, especially in font-heavy or resource-intensive web projects.

The font-display Property

The font-display property in CSS is part of the @font-face rule and controls how a browser handles the display of text when a web font is loading or fails to load. This property is particularly useful for improving perceived performance and user experience when using custom web fonts.

Syntax

The font-display property is defined inside an @font-face rule:

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

Values of font-display

1. auto (Default):

   • The browser decides how the font should be displayed during the font-loading process.
   • Behavior can vary between browsers, but it generally attempts to balance performance and user experience.

   Example:

   font-display: auto;
   

2. block:

   • The browser hides the text (invisible text) for a brief period while the font is loading.
   • If the font loads within the block period (usually around 3 seconds), the text is displayed using the web font.
   • If it doesn’t load in time, the browser switches to a fallback font.

   Effect:

   • Good for preserving design consistency (no fallback font used initially).
   • Bad if the block period is too long, as users see no text at all.

   Example:

   font-display: block;
   

3. swap:

   • The browser displays the text immediately using a fallback font.
   • Once the web font loads, it swaps the fallback font for the web font.

   Effect:

   • Good for fast content visibility (minimizes invisible text).
   • Bad if the transition between fallback and web font is noticeable (can cause a layout shift or "flash of unstyled text" – FOUT).

   Example:

   font-display: swap;
   

4. fallback:

   • The browser displays the text immediately using a fallback font.
   • If the web font doesn’t load quickly (after a short timeout period, usually about 100ms), the fallback font remains in use, and the web font will not be applied.

   Effect:

   • Good for prioritizing speed and avoiding font swaps.
   • Bad if the fallback font looks significantly different from the intended web font.

   Example:

   font-display: fallback;
   

5. optional:

   • The browser displays the text immediately using a fallback font.
   • The web font will only be used if it loads almost immediately and doesn't interfere with performance or bandwidth usage (e.g., over slow connections).

   Effect:

   • Good for low-priority fonts on resource-constrained networks.
   • Bad if you rely heavily on the web font for branding or design consistency.

   Example:

   font-display: optional;
   

Visual Timeline for font-display

Choosing the Right Value

• swap is ideal when performance and content visibility are critical. It ensures that users see content immediately, even if the web font hasn’t loaded yet.
• block is suitable for cases where visual consistency is crucial and users can tolerate a brief delay.
• fallback and optional work best when bandwidth is limited or when fallback fonts are close in appearance to the web font.
• auto can be used when you're unsure or want to leave the decision to the browser.