The Role of Third-Party Scripts and Ads in Causing Layout Shifts

Third-party scripts and ads play a significant role in causing layout shifts on web pages, primarily because they load content asynchronously and often without reserved space, which leads to unexpected movement of visible elements.

Key points explaining their role include:

• Asynchronous Loading and DOM Manipulation: Third-party scripts, such as those from analytics tools or ad services, often load content after the initial page render. This can manipulate the Document Object Model (DOM) dynamically, causing elements to shift position unexpectedly.

• Ads Loading Without Reserved Space: Ads are typically requested asynchronously and inserted dynamically. If the page does not reserve sufficient space for these ads beforehand, when the ads finally load, they push down or shift existing content, causing layout instability. This is especially problematic for ads placed near the top of the viewport, as they displace more visible content and create a more noticeable shift.

• Embeds and iFrames: Third-party embeds like videos, social media widgets, or maps often do not specify fixed dimensions before loading. This omission causes the page layout to adjust once the embed content loads and its size becomes known, resulting in layout shifts.

• Dynamic Content Changes: Some third-party scripts insert or expand content dynamically (e.g., pop-ups, expanding ads), which can cause further layout shifts if not managed properly.

• Font Loading Effects: Third-party font loading strategies can cause text to reflow when fonts are applied, contributing to layout shifts.

To mitigate these issues, best practices include:

• Reserving Fixed Space: Define fixed container sizes or placeholders for ads and embeds before they load, so the browser allocates space and prevents content from shifting.

• Using Tools and Plugins: Employ tools like Layout Shift Terminator to analyze and set appropriate container sizes for embeds, or use ad management plugins that support placeholder containers with set dimensions.

• Careful Placement: Avoid placing ads or embeds directly above critical content areas where shifts would be most disruptive.

In summary, third-party scripts and ads cause layout shifts mainly due to their asynchronous loading and lack of reserved space, which leads to visible content moving unexpectedly. Proper space reservation and management strategies are essential to minimize these shifts and improve user experience.