Introduction to Layout Flashes and Their Impact

Layout flashes, also known as Flash of Unstyled Text (FOUT) or Flash of Unavailable Font (FOIT), are a common issue that can negatively impact user experience on websites. When a website loads, it may briefly display text using a default font before the custom web font is loaded, causing a flash that can be distracting and unprofessional. This issue is particularly prevalent in websites that rely heavily on custom fonts for their design and branding.

Understanding Layout Flashes

Layout flashes occur when the browser needs to render text with a custom font but the font file has not yet been downloaded. The browser will initially render the text using a default font, such as sans-serif or serif, and once the custom font is loaded, it will replace the default font. This brief flash can be subtle or more noticeable, depending on the font and the size of the text.

FOIT (Flash of Unavailable Font) is the most common type of layout flash, where the browser waits for the custom font to be fully loaded before rendering any text. This can result in a longer delay before the text becomes visible. On the other hand, FOUT (Flash of Unstyled Text) occurs when the browser renders the text with the default font but the custom font is still loading, which can cause a brief flicker or shift in the layout.

Why Web Fonts Can Cause Layout Flashes

Web fonts are a powerful tool for enhancing the visual appeal of a website, but they can also introduce performance issues if not implemented correctly. The primary reason web fonts can cause layout flashes is because they are loaded asynchronously, which means the browser has to wait for the font file to be downloaded before it can render the text with that font.

When a web font is requested, the browser will initiate a request to the font server. If the font is not cached on the user's device, it will need to be downloaded, which can take time. During this time, the browser will render the text using the default font, which can create a layout flash. This can be especially noticeable on slow connections or on devices with limited processing power.

How to Prevent Layout Flashes with Web Fonts

1. Use the font-display Property

One of the most effective ways to prevent layout flashes is to use the font-display property in your CSS. This property allows you to control how the browser displays text while the font is being loaded. The font-display property has several values, each with different behaviors:

  • auto: The browser will use the default behavior, which is typically FOIT. This can result in a layout flash.
  • block: The browser will block rendering of text until the font is fully loaded. This can prevent layout flashes but may result in a longer delay before the text becomes visible.
  • swap: The browser will render the text with the default font while the font is being loaded, and once the font is loaded, it will swap out the default font for the custom font. This can prevent layout flashes but may cause a brief flicker.
  • fallback: The browser will render the text with the default font while the font is being loaded. If the font fails to load, it will switch to the fallback font. This can help prevent layout flashes while also providing a fallback option.
  • hidden: The browser will not render the text at all until the font is fully loaded. This can prevent layout flashes but may result in a poor user experience if the text is critical.

Using the swap or fallback values can help prevent layout flashes while still providing a good user experience. The swap value is often the best choice for most websites, as it allows the text to be visible immediately while the font is being loaded.

2. Use a Fallback Font

Another important step in preventing layout flashes is to use a fallback font. A fallback font is a default font that the browser will use if the custom font fails to load. This can help prevent layout flashes and ensure that the text remains readable even if the custom font is not available.

To use a fallback font, you can specify it in your @font-face rule or in the font-family property of your CSS. For example:

body {
  font-family: 'MyCustomFont', sans-serif;
}

In this example, if the MyCustomFont fails to load, the browser will use the sans-serif fallback font. This can help prevent layout flashes and ensure that the text remains readable.

3. Load Fonts Asynchronously

Web fonts are typically loaded asynchronously, which means that the browser will download the font file in the background while the rest of the page is being rendered. However, this can sometimes lead to layout flashes if the font is not loaded before the text is rendered.

To ensure that fonts are loaded asynchronously, you can use a font loading strategy that allows the browser to render the text with a default font while the font is being loaded. This can be achieved by using the font-display property with the swap value, as discussed earlier.

Additionally, you can use the Web Font Loader library to manage the loading of web fonts. This library allows you to specify a fallback font and control how the font is loaded and displayed. This can be particularly useful for websites that use multiple custom fonts.

4. Optimize Font Files

Optimizing font files is another important step in preventing layout flashes. Large font files can take longer to download, which can increase the likelihood of layout flashes. To optimize font files, you can use tools such as Font Squirrel or Google Fonts to generate optimized font files that are suitable for web use.

Optimizing font files can also help reduce the overall size of the font file, which can improve the performance of your website. This can be particularly beneficial for websites that have a large number of custom fonts or that are used on mobile devices with limited bandwidth.

5. Use Font Subsetting

Font subsetting is a technique that involves only including the characters that are actually used on your website. This can help reduce the size of the font file and improve the performance of your website.

For example, if your website only uses the Latin alphabet and some common symbols, you can subset the font to include only those characters. This can significantly reduce the size of the font file and improve the loading time of your website.

6. Use a Font Preloader

A font preloader is a technique that allows you to load font files before they are needed. This can help prevent layout flashes by ensuring that the font is already loaded when the text is rendered.

To use a font preloader, you can use the preload attribute in your link tag. For example:

<link rel="preload" href="myfont.woff2" as="font" type="font/woff2" crossorigin="anonymous">

This will tell the browser to preload the font file, which can help ensure that the font is available when the text is rendered. This can be particularly useful for websites that use large font files or that have a large number of custom fonts.

7. Use a Font Loading Strategy

A font loading strategy is a set of techniques that you can use to manage the loading of web fonts on your website. This can include using the font-display property, using a fallback font, and using a font preloader.

By implementing a font loading strategy, you can ensure that your website loads quickly and that the user experience is not negatively impacted by layout flashes. This can be particularly important for websites that rely heavily on custom fonts for their design and branding.

Conclusion

Layout flashes can be a frustrating issue for users, but with the right strategies, you can prevent them and ensure a smooth user experience on your website. By using the font-display property, using a fallback font, loading fonts asynchronously, optimizing font files, using font subsetting, and implementing a font loading strategy, you can ensure that your website loads quickly and that the text is displayed correctly without any layout flashes.

Remember, the key to preventing layout flashes is to balance between speed and visual appeal. By carefully managing the loading of web fonts, you can ensure that your website provides a great user experience while also maintaining a professional and visually appealing design.