Cumulative Layout Shift (CLS) and Optimization Strategies

What Is Cumulative Layout Shift (CLS)?

Cumulative Layout Shift (CLS) is a metric that measures the visual stability of a webpage. CLS quantifies the amount of unexpected layout shifts that occur while a page is loading. A high CLS score indicates that elements on the page often shift during loading, which can be disorienting for users and detract from their browsing experience.

Why Is CLS Important?

CLS is crucial because unstable layout shifts can:

• Disrupt users trying to read or click on something.
• Reduce the credibility of your website as it appears less professional.
• Impact SEO ranking, as Google considers CLS within its Core Web Vitals.

If your website has a poor CLS score, users are likely to leave the page before finding the information they need. Additionally, a website with a good CLS score will have a competitive edge in SEO, as Google increasingly prioritizes user experience.

How to Measure CLS

Methods for Measuring CLS

To measure CLS, you can use various tools, including:

1. Google PageSpeed Insights: This tool provides a CLS score as part of your website's overall performance report.
2. Google Search Console: Offers CLS data within the Core Web Vitals section.
3. Lighthouse: This tool, available in Google Chrome, provides detailed insights into CLS.
4. Chrome User Experience Report (CrUX): Provides CLS data based on real user experiences.

An ideal CLS score is below 0.1. A score above 0.25 indicates that your website has significant layout shift issues.

Common Causes of Cumulative Layout Shift

CLS is often caused by elements that do not have defined dimensions or elements that appear dynamically after the page has started loading. Here are some common causes of CLS:

1. Images Without Dimensions: If you add images without specifying width and height, the page layout will shift when the images load.
2. Slow Loading Web Fonts: When fonts load slowly, text can jump as the default font is replaced by the custom font.
3. Ads or Dynamic Content: Elements like ads that load dynamically can cause layout shifts as they appear or disappear.
4. Embedded Videos or External Elements Without Fixed Sizes: These elements can often cause shifts if their dimensions are not defined.
5. Animations or Transitions: Poorly configured animations or transitions can cause elements to move around as they load.

Strategies to Optimize Cumulative Layout Shift

1. Set Dimensions for Images and Videos

To prevent layout shifts, always set the dimensions (width and height) for all images and videos on your website. This ensures that the browser can reserve the appropriate space for these elements, even before they fully load.

2. Use Font Display Property

If you use custom fonts, be sure to use the font display property. For instance, apply font-display: swap; to ensure that the default font is displayed immediately while the custom font loads in the background. This reduces text jumping that can cause CLS.

3. Avoid Using Dynamic Elements Above the Fold

Ensure that ads, widgets, or other dynamic content are not placed above the fold (the part of the page users see first without scrolling). These elements often cause layout shifts when they load or change size.

4. Reserve Space for Ads or Dynamic Elements

If you need to include ads or other dynamic elements on the page, make sure to reserve enough space for them. This can be done by setting a minimum size or using placeholders with fixed dimensions.

5. Optimize CSS and JavaScript

Inefficient CSS and JavaScript can cause elements to move during loading. Ensure that you:

• Optimize CSS by removing unnecessary code and minimizing file size.
• Defer the loading of non-essential JavaScript to prevent layout shifts caused by slow rendering.

6. Use Lazy Loading

Lazy loading allows elements like images and videos to load only when users scroll near them. This can help reduce CLS, particularly if large elements are not needed immediately on the page.

Case Study: Improving CLS for an E-commerce Website

E-commerce websites often have dynamic elements like recommended products, ads, and image carousels. Here are the steps taken by an e-commerce site to improve their CLS score:

1. Setting Dimensions for All Product Images: The e-commerce team added dimensions to all product images, including those in the carousel, to avoid layout shifts.
2. Implementing Lazy Loading for Below-the-Fold Images: They applied lazy loading for images not visible immediately when the page loads.
3. Optimizing CSS and JavaScript: By minimizing CSS files and deferring non-essential JavaScript, they reduced shifts caused by slow rendering.

As a result, they managed to reduce their CLS score from 0.3 to 0.05, significantly improving user experience and SEO ranking.

FAQ about Cumulative Layout Shift (CLS)

1. What is CLS, and why is it important for SEO?
CLS measures layout shifts during page loading. Google considers CLS in its ranking algorithm because it affects user experience.

2. What is the ideal CLS score?
The ideal CLS score is below 0.1. Scores between 0.1 and 0.25 require improvement, and scores above 0.25 indicate optimization is necessary.

3. How do I measure CLS?
You can measure your site’s CLS using tools like Google PageSpeed Insights, Google Search Console, or Lighthouse.

4. What causes a high CLS score?
A high CLS score is often caused by images without dimensions, slow-loading fonts, dynamic elements like ads, and embedded videos or external elements without fixed sizes.

5. How can I reduce my website’s CLS score?
To reduce your CLS score, ensure you set dimensions for images and videos, use font display properties for custom fonts, and reserve space for dynamic elements.

6. Does CLS affect only desktop sites?
No, CLS affects both desktop and mobile sites. Therefore, CLS optimization should be applied to both versions of your site.

7. Can lazy loading help reduce CLS?
Yes, lazy loading can help reduce CLS because elements that are not immediately visible when the page loads are deferred until necessary.

8. Do animations affect CLS?
Poorly configured animations can cause elements to shift and contribute to CLS. However, when properly configured, animations can enhance user experience without impacting CLS.

By understanding and optimizing CLS, you can significantly improve user experience and boost your website’s SEO performance. If you are serious about improving site performance, start with the steps above to ensure that your site offers a stable and comfortable user experience.