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How to Improve CLS on Mobile Pages: A Step-by-Step Guide

How to Improve CLS on Mobile Pages: A Step-by-Step Guide

Author

Date Published

October 24, 2025

Table of contents

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To improve Cumulative Layout Shift (CLS) on mobile pages, optimize images and fonts, reserve space for dynamic content, avoid inserting elements above existing content, and leverage proper CSS and performance best practices.

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Key Takaways

CLS measures unexpected visual shifts — a poor mobile CLS score causes user frustration and lower page rankings.
Always define width and height for images and videos to prevent layout movement as media loads.
Preload web fonts and use fallback fonts to minimize layout changes during font rendering.
Avoid dynamically injecting content like ads or banners above main content without proper space reservation.
Use transform animations instead of layout-triggering changes like modifying height or margin.
Test your mobile CLS using tools like Google PageSpeed Insights and Lighthouse for targeted improvements.

Frequently Asked Questions

What is a good CLS score for mobile?

CLS is part of Google's Core Web Vitals, which influence mobile search rankings and user experience. A low CLS improves engagement and lowers bounce rates.

Why is CLS important for mobile SEO?

CLS is part of Google's Core Web Vitals, which influence mobile search rankings and user experience. A low CLS improves engagement and lowers bounce rates.

Can fonts cause layout shifts on mobile?

Yes, late-loading custom fonts can cause text layout shifts. Preloading fonts and using font-display: swap helps prevent this.

How do ads impact CLS?

Ads often load after content and push elements down. Reserving space for ad containers avoids shifting.

What tools can I use to track CLS?

Use Google PageSpeed Insights, Chrome DevTools Lighthouse, or the Web Vitals extension to track and debug CLS values.

Step by Step Plan

01

Measure Your Current CLS Score

Use tools like Google PageSpeed Insights or Lighthouse to identify layout shifts and quantify your CLS value. Aim for a score below 0.1 on mobile.

02

Set Dimensions for Images and Media

Explicitly define width and height attributes for all images, videos, and embeds to reserve space before content loads.

03

Preload Fonts and Use Font Display Settings

Use font-display: optional or swap in CSS and preload custom fonts to prevent large layout shifts during rendering.

04

Preload Fonts and Use Font Display Settings

Allocate fixed-size containers for elements like ads, pop-ups, or third-party widgets to reduce unexpected visual jumps.

05

Avoid Above-the-Fold DOM Manipulation

Prevent layout shifts by minimizing JavaScript-injected elements above existing content, especially during page load.

Comparison Table

Mobile Page with Poor CLS
Mobile Page with Optimized CLS
Images load without reserved space, causing content to jump
Image dimensions defined to prevent movement
Ads inject above content after load
Ads have reserved blank containers
Fonts load slowly and re-render text
Fonts are preloaded with fallback styles
Animations modify layout dimensions
Uses transform and opacity for animations
User frustration with shifting content
Stable, user-friendly mobile experience

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Sources

Web.dev: Cumulative Layout Shift Explained
Google PageSpeed Insights
Web.dev: Controlling Font Performance with font-display

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