Tackling CLS: Enhancing Visual Stability on the Web
Cumulative Layout Shift (CLS) has become a fundamental metric in assessing the visual stability of web pages. This blog post explores the concept of CLS, its importance, and strategies to optimize it for a better user experience.
Understanding Cumulative Layout Shift (CLS)
CLS measures the sum total of all individual layout shift scores for every unexpected layout shift that occurs during the lifespan of a page. A layout shift happens when visible elements on the page change position or size, causing a jarring and disorienting experience for users.
Why Does CLS Matter?
High CLS can lead to a poor user experience. Imagine reading an article and suddenly the text moves, or clicking a link just as it shifts, leading to a wrong action. CLS quantifies these frustrating experiences, making it an essential metric for user-centric web design.
How CLS is Calculated
The CLS score is calculated based on two factors:
- Impact Fraction: The portion of the viewport affected by the shift.
- Distance Fraction: The distance the elements have moved, relative to the viewport.
CLS Scores Interpretation
- Good: A CLS score of 0.1 or less is considered good.
- Needs Improvement: Scores between 0.1 and 0.25 need optimization.
- Poor: Scores above 0.25 are deemed poor and require significant improvements.
Strategies for Reducing CLS
Minimizing CLS is crucial for a seamless web experience. Here are some effective strategies:
Reserve Space for Dynamic Content
Allocating space for ads, embeds, or images before they load can prevent shifts. This can be done by setting explicit width and height attributes.
Avoid Inserting New Content Above Existing Content
Adding new content, like a live feed update, above existing content can cause shifts. Instead, append new content below or offer a user interaction to load more.
Use Transform Animations
For animations or transitions, prefer CSS transform properties. These properties don’t affect the layout of the page, reducing the likelihood of layout shifts.
Load Fonts Efficiently
Unloaded web fonts can cause text to shift when they load. Implementing font-display strategies can mitigate this.
Tools for Measuring CLS
Tools like Google’s Lighthouse, PageSpeed Insights, and Chrome DevTools can help measure and debug CLS, offering insights and recommendations for improvements.
Real User Monitoring (RUM)
RUM tools capture actual user experiences, providing real-world CLS data which is crucial for understanding how users are impacted by layout shifts on different devices and network conditions.
Optimizing for CLS is not just about improving a score; it’s about enhancing the user experience by providing a stable and predictable visual journey. By addressing CLS, developers and designers can significantly reduce user frustration and improve the overall effectiveness of their websites.