Strategies for Optimizing Cumulative Layout Shift (CLS) in Frontend Applications

Strategies for Optimizing Cumulative Layout Shift (CLS) in Frontend Applications

1. Problem Description

Cumulative Layout Shift (CLS) is a core metric of Core Web Vitals, used to measure the visual stability of a page. It quantifies the frequency and extent of unexpected layout shifts that users see during the loading process. Examples include: images suddenly occupying space after lazy loading, dynamically inserted ads pushing content down, etc. A lower CLS score indicates a more stable user experience.

2. How CLS is Calculated

CLS calculates an "impact score" for each layout shift using the following formula:
CLS = Impact Fraction × Distance Fraction

  • Impact Fraction: The proportion of the viewport area occupied by the shifting element (0~1).
  • Distance Fraction: The proportion of the viewport's largest dimension (height or width) that the element moves within the viewport (0~1).

For example: An image occupying 30% of the viewport height moves down by 10% of the viewport height. The CLS for this shift = 0.3 × 0.1 = 0.03. The cumulative value of all shifts during the page's lifecycle is the final CLS score.

3. Common Scenarios Causing High CLS

  1. Images or Videos Without Dimensions: They occupy space after loading, pushing down surrounding content.
  2. Dynamically Injected Content: Non-user-initiated elements like ads or pop-ups.
  3. Layout Changes Due to Font Loading: Size differences between fallback fonts and intended web fonts.
  4. Asynchronously Loaded Components: Such as comment sections, recommendation widgets, etc.

4. Optimization Strategies & Implementation Steps

Step 1: Preset Dimensions for Media Elements

  • Principle: Reserve the correct space before images/videos load to prevent layout jumps.
  • Methods:
    • Explicitly set width and height attributes (or CSS dimensions) for <img> and <video> elements.
    • Use CSS Aspect Ratio Boxes for responsive adaptation: 
      .img-container {
  width: 100%;
  height: 0;
  padding-top: 56.25%; /* 16:9 Aspect Ratio */
  position: relative;
}
.img-container img {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}

Step 2: Avoid Shifts from Dynamically Injected Content

  • Principle: Ensure dynamically added content does not push existing elements.
  • Methods:
    • Reserve space for dynamic content containers in advance (e.g., using min-height).
    • Use transform animations instead of layout-affecting properties like top/left.
    • Use fixed-size containers for ad slots to prevent page stretching after load.

Step 3: Optimize Layout Shifts Caused by Font Loading

  • Principle: Minimize rendering differences between fallback fonts and target web fonts.
  • Methods:
    • Use font-display: optional or swap combined with font-size-adjust to fine-tune font proportions.
    • Preload critical fonts using <link rel="preload">: 
      <link rel="preload" href="font.woff2" as="font" type="font/woff2" crossorigin>
    • Employ a FOIT (Flash of Invisible Text) strategy to hide text until the font loads.

Step 4: Use CSS Properties to Enforce Layout Stability

  • Principle: Use CSS to restrict the potential for element shifts.
  • Methods:
    • Add transform: translateZ(0) to potentially shifting elements to promote them to their own layer.
    • Use content-visibility: auto for dynamic content to defer rendering of off-screen regions.

Step 5: Monitoring and Debugging CLS

  • Tools:
    • Chrome DevTools Performance panel records layout shifts (shown as Layout Shift entries).
    • Actively monitor CLS using PerformanceObserver: 
      new PerformanceObserver((entryList) => {
  for (const entry of entryList.getEntries()) {
    console.log('CLS Shift:', entry);
  }
}).observe({type: 'layout-shift', buffered: true});
    • Lighthouse or the web-vitals library to directly obtain CLS scores.

5. Summary

The core of CLS optimization is predictive layout: Ensure page elements remain stable during rendering by presetting dimensions, reserving space, and controlling asynchronous loading. Combining tool-based monitoring of actual shifts with targeted adjustments to key elements can significantly improve visual stability, ultimately meeting the Core Web Vitals 'good' threshold for CLS (less than 0.1).