You click a button, but the page moves at the last second and you end up selecting something else. Most internet users have experienced this frustration, even if they didn't know what caused it. Understanding what causes layout shifts on websites has become increasingly important as site owners focus on user experience, performance, and Google's Core Web Vitals.
Why Layout Shifts Have Become a Bigger Issue
Websites have changed dramatically over the past decade. Pages are no longer simple collections of text and images. They now contain advertisements, embedded videos, live chat widgets, recommendation engines, social media feeds, and countless third-party scripts.
All of these elements compete for space on a page.
When a browser cannot determine where content belongs before it loads, elements start moving around unexpectedly. What seems like a minor visual annoyance can quickly become a serious usability problem.
For visitors, layout shifts create uncertainty. For businesses, they can lead to lower engagement, reduced conversions, and weaker performance metrics. That's why Google introduced Cumulative Layout Shift (CLS) as part of its Core Web Vitals initiative.
Understanding What a Layout Shift Actually Is
A layout shift occurs when visible content changes position unexpectedly after the page has already begun rendering.
The key word is "unexpectedly."
If a user clicks a button that intentionally expands a menu, that movement isn't considered a layout shift. The user initiated the action and expects a change.
Problems arise when content moves without warning.
Imagine reading an article when a banner suddenly appears above the text. Everything shifts downward. You lose your place and have to find it again. That is a classic example of a layout shift.
The larger the movement and the more frequently it occurs, the worse the experience becomes.
Images Without Reserved Space
One of the oldest causes of layout instability remains one of the most common.
When an image is added to a page without dimensions, the browser doesn't know how much space it should reserve. As a result, surrounding content loads first. Once the image arrives, the browser inserts it into the layout and pushes everything else aside.
This problem appears frequently on blogs, news websites, and e-commerce stores.
A product image that loads late can move pricing information. A featured image in a blog post can push paragraphs lower on the page. Large image galleries often create even bigger disruptions.
Modern browsers handle image sizing better than they once did, but developers still need to provide proper dimensions or aspect ratios. Without them, the browser is left guessing.
Why Advertisements Create So Many Layout Shifts
Publishers often discover that advertising is responsible for a significant portion of their CLS issues.
Unlike static content, advertisements are dynamic. The final ad size may not be known when the page initially loads. Ad networks select creative assets in real time, and those assets can vary considerably.
A page may reserve a small area for an advertisement only to receive a larger creative a few moments later. The browser then needs to rearrange nearby elements to make room.
This issue becomes particularly noticeable on mobile devices. A movement that seems minor on a desktop screen can occupy a large portion of a smartphone display.
Many publishers face a difficult balancing act. Advertising revenue matters, but so does user experience. The most successful sites reserve space for likely ad sizes before ads load rather than waiting for the final creative to arrive.
Embedded Content Often Arrives Late
External content presents another challenge.
YouTube videos, Google Maps integrations, social media posts, review widgets, and booking tools all rely on external servers. The browser frequently has limited information about their final dimensions during the initial page render.
The result is familiar. The page appears stable at first. Then an embedded element finishes loading and forces surrounding content to shift.
A travel website may embed maps throughout destination guides. A marketing blog may include social media posts within articles. An online store may display customer reviews through a third-party platform.
Each integration introduces another opportunity for layout instability.
The more external services a website depends on, the more important it becomes to reserve space before those elements load.
Web Fonts Can Quietly Cause Problems
Many website owners focus on images and advertisements while overlooking typography.
Custom fonts help establish a brand's identity, but they can also contribute to layout shifts.
When a page loads, browsers often display a temporary fallback font before downloading the preferred typeface. If the replacement font has different spacing, width, or height characteristics, the text may reflow when the new font appears.
The effect is usually subtle, yet subtle shifts occurring across multiple sections of a page can add up quickly.
Headlines tend to reveal the problem most clearly. A large heading may occupy two lines using one font and three lines using another. That small difference can push every element beneath it downward.
Developers who understand font metrics and loading behavior can reduce these issues considerably.
Dynamically Added Content Can Disrupt Entire Pages
Some layout shifts happen long after the page appears fully loaded.
Website owners often add promotional banners, newsletter invitations, discount announcements, cookie notices, and alert messages. While these features serve legitimate purposes, their implementation matters.
A common mistake involves inserting content at the top of a page after users have already started interacting with it.
Picture someone reading the third paragraph of an article. Suddenly a promotional banner appears above the headline. Everything moves.
The user hasn't done anything wrong. The website simply changed the layout beneath them.
This type of interruption often generates more frustration than slower loading times because it occurs while people are actively engaged with content.
The Hidden Risks of Lazy Loading
Lazy loading has become a standard performance technique. Instead of loading every image immediately, websites wait until users approach the content.
The strategy works well when implemented correctly.
Problems emerge when placeholders fail to reserve sufficient space.
As visitors scroll, images begin loading. If the browser hasn't already allocated space for them, surrounding content shifts to accommodate the new elements.
Many websites unintentionally trade one performance issue for another. They improve loading speed while damaging visual stability.
The goal should never be speed alone. A fast website that constantly shifts around still creates a poor experience.
Successful implementations balance both priorities.
Mobile Devices Often Reveal Problems First
A layout that seems perfectly stable on a desktop monitor may behave very differently on a smartphone.
Smaller screens leave less room for error. Elements stack vertically, advertisements occupy a larger percentage of the viewport, and responsive layouts must adapt to changing dimensions.
Even minor shifts become noticeable.
A navigation menu expanding unexpectedly on a desktop screen might be irritating. The same movement on a phone can cover most of the visible content.
This is one reason many website owners are surprised by poor CLS scores. Their desktop experience appears fine, yet mobile users encounter constant visual instability.
Testing across multiple devices remains essential because layout shifts rarely affect every visitor in the same way.
How Layout Shifts Affect SEO and Business Performance
Google's interest in layout stability isn't arbitrary.
Users consistently prefer websites that feel predictable and easy to navigate. When pages jump around unexpectedly, trust begins to erode.
A visitor trying to complete a purchase may click the wrong button. Someone reading an article may lose their place. A form submission may become more difficult than necessary.
These frustrations influence behavior.
People spend less time on unstable websites. They abandon pages more frequently. They become less likely to complete important actions.
Search engines pay attention to these signals.
While great content remains the foundation of search visibility, technical quality contributes to the overall experience. Websites that combine strong content with stable performance tend to create better outcomes for both users and businesses.
Finding and Fixing Layout Shift Problems
Many layout shifts occur too quickly to notice during casual browsing. Fortunately, several tools can identify them.
Google PageSpeed Insights highlights CLS issues and points toward potential causes. Lighthouse audits provide detailed performance reports. Chrome DevTools allows developers to observe shifts as they happen.
The most effective fixes usually focus on a handful of recurring issues:
Reserve space for images before they load. Allocate dimensions for embedded content. Leave room for advertisements. Optimize font loading behavior. Avoid inserting new content above existing content. Review third-party scripts regularly.
Rarely does a website suffer from a single catastrophic problem. More often, numerous small issues accumulate over time. Addressing them systematically produces meaningful improvements.
Conclusion
The answer to what causes layout shifts on websites is rarely limited to one factor. In most cases, the problem stems from content that arrives after the browser has already built the initial page structure. Images without dimensions, dynamically loaded advertisements, embedded media, custom fonts, lazy loading mistakes, and third-party scripts all contribute to unexpected movement.
Layout shifts may seem like a technical detail, but visitors experience them directly. Every jump, flicker, or unexpected movement interrupts the browsing experience. Websites that prioritize visual stability feel more professional, easier to use, and ultimately more trustworthy. As user expectations continue to rise, reducing layout shifts is no longer just a performance task. It has become a fundamental part of building a better website.
