Understanding Html Reflow and Its Impact on Page Speed

Reflown happens when the browser recalculates the positions and geometries of elements within a web page for re-rendering. Learn how this affect pagespeed.

Arjen Karel Core Web Vitals Consultant
Arjen Karel

Understanding Web Reflow and Its Impact on Page Speed

Reflow, also known as layout or recalculation, is a crucial process in web browsers that recalculates the positions and geometries of elements within a web page for re-rendering. This process is essential for displaying web content correctly, but it can also significantly affect page speed and overall performance. In this article, we will explore what web reflow is, its triggers, and how it influences pagespeed based on the provided information.

What is Reflow?

Web reflow is the process where the web browser calculates the layout of a web page, including the positioning and size of each element, based on the underlying Document Object Model (DOM) and CSS styles. Whenever there are changes to the DOM or CSS that affect the layout, the browser needs to perform reflow to update the page's appearance correctly.

Triggers for Web Reflow:

Several actions can trigger web reflow, including user interactions and Dynamic HTML (DHTML) changes. Common triggers include:

  1. Changes to DOM Elements: Adding, removing, or modifying elements within the DOM can cause reflow, as the browser must recalculate the layout to accommodate these changes.

  2. CSS Modifications: Altering CSS properties such as width, height, margin, padding, or transforms can trigger reflow, as the browser needs to adjust the element's positioning and geometry accordingly.

  3. Font Loading: When web fonts are loaded or updated, it can affect the layout of the text, leading to reflow.

  4. Window Resizing: Changing the browser window's size forces reflow as the layout needs to adapt to the new dimensions.

  5. Changes in Media Queries: When the screen size or orientation changes, the browser may recalculate the layout based on the new media query rules.

Impact of Web Reflow on PageSpeed:

Web reflow can have a significant impact on page speed and overall user experience. The process of recalculating the layout and rendering the page takes time and computational resources, which can lead to slower loading times and reduced performance. Here's how web reflow affects pagespeed:

  1. Performance Bottleneck: Excessive and frequent reflows can create a performance bottleneck, leading to slower page rendering and a suboptimal user experience.

  2. Layout Thrashing: Layout thrashing occurs when the browser performs multiple unnecessary reflows due to frequent DOM updates. This can lead to janky animations and a choppy user interface.

  3. Cumulative Layout Shift (CLS): CLS measures the visual stability of a web page by calculating unexpected layout shifts during the page load. Frequent reflows can contribute to a higher CLS score, negatively impacting SEO and user experience.

Minimizing Browser Reflow for Better PageSpeed:

To optimize pagespeed and minimize the impact of web reflow, developers should follow some best practices:

  1. Reduce DOM Depth: Keep the DOM structure shallow and avoid nesting too many elements within each other. A flatter DOM tree leads to faster reflows.

  2. Optimize CSS Rules: Limit the use of complex CSS selectors and avoid applying unnecessary styles. Reduce the number of CSS rules to minimize the recalculation overhead.

  3. Batch DOM Modifications: When making multiple DOM changes, batch them together to reduce the number of reflows. This can be achieved using techniques like requestAnimationFrame or using DocumentFragment to insert multiple elements.

  4. Use Transforms and Transitions: When animating elements, prefer CSS transforms and transitions instead of properties like width and height, as transforms are more efficient and less likely to trigger reflow.

  5. Optimize Web Fonts: Carefully select and optimize web fonts to minimize their impact on reflow and loading times.

Understanding Interaction to Next Paint (INP):

The Interaction to Next Paint (INP) is a metric that assesses a webpage's responsiveness to user interactions. It measures the time taken by the browser to process and display visual feedback in response to user actions like clicks, taps, and key presses. A low INP score indicates a quick and smooth response, while higher scores indicate poor responsiveness, leading to potential confusion and unsatisfactory user experiences.

INP is an essential Core Web Vitals metric that provides valuable insights into how users perceive a website's interactivity. Scores below 200 milliseconds are considered good, while those above 500 milliseconds indicate the need for improvement in responsiveness.

The Connection Between Reflow and INP:

Web reflow plays a significant role in determining INP scores. When user interactions trigger changes in the webpage's layout or styling, the browser needs to recalculate the positions and geometries of the affected elements. Reflow can be a resource-intensive operation, impacting the time it takes for the next paint event to occur.

As the browser performs reflow during user interactions, the delay caused by reflow can lead to a higher INP score. Excessive and frequent reflows can create performance bottlenecks, resulting in slower responsiveness to user actions, leading to a poorer INP score.

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Understanding Html Reflow and Its Impact on Page SpeedCore Web Vitals Understanding Html Reflow and Its Impact on Page Speed