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Cumulative Layout Shift (CLS) is a key metric in web performance that measures visual stability. High CLS scores can frustrate users, especially on websites that heavily rely on custom fonts and icons. Addressing CLS effectively ensures a smoother user experience and better SEO rankings.
Understanding the Causes of CLS with Custom Fonts and Icons
Custom fonts and icons often load asynchronously, which can cause layout shifts. When fonts or icons load late, text and images may reflow, leading to a poor user experience. Common causes include:
- Unspecified font sizes or fallback fonts that change once custom fonts load.
- Late-loading icon fonts that shift content once they appear.
- Images or icons without reserved space.
Strategies to Minimize CLS
1. Use Font Display Swap
Implement font-display: swap in your @font-face CSS. This allows text to be visible immediately using fallback fonts, then swaps to custom fonts once loaded, preventing layout shifts.
2. Reserve Space for Fonts and Icons
Specify width and height attributes for images and icons. For fonts, set a fallback font with similar size and style to reduce reflow when custom fonts load.
3. Optimize Font and Icon Loading
Use preload links in the <head> to prioritize loading of critical fonts and icons. This reduces delay and minimizes shifts.
Additional Tips for Better Stability
Combine font optimization with other best practices:
- Use a Content Delivery Network (CDN) to speed up font and icon delivery.
- Limit the number of web fonts and icon sets to reduce load times.
- Implement CSS containment to isolate layout changes.
By applying these strategies, you can significantly reduce CLS caused by heavy use of custom fonts and icons, resulting in a more stable and enjoyable website experience.