#atom

Subtitle:

A user experience metric measuring visual stability during page loading

Core Idea:

Cumulative Layout Shift (CLS) measures the sum of all unexpected layout shifts that occur during the entire lifespan of a webpage, quantifying visual stability and preventing frustrating user experiences.

Key Principles:

1. Visual Stability Measurement:
   • Tracks elements that unexpectedly change position after becoming visible to users.
2. Weighted Impact Calculation:
   • Considers both the size of the unstable element and the distance it moves when calculating the impact score.
3. Lifecycle Accumulation:
   • Accumulates all layout shifts throughout the page lifecycle, not just during initial load.

Why It Matters:

• Frustration Reduction:
  • Prevents misclicks when elements shift as users attempt to interact with the page.
• Reading Experience:
  • Maintains readability by preventing text from jumping while users are engaged with content.
• Professional Perception:
  • Sites with minimal layout shift appear more polished and trustworthy to users.

How to Implement:

1. Set Size Attributes on Media:
   • Always include width and height attributes on images, videos, and other media elements.
2. Reserve Space for Dynamic Content:
   • Pre-allocate space for ads, embeds, and dynamically loaded content using placeholders.
3. Manage Font Loading:
   • Implement proper font loading strategies with size-adjust properties to prevent text shifts.

Example:

• Scenario:

  • A blog with multiple images and web fonts throughout the article content.

• Application:

  • The developer implements proper image dimensions and font loading optimization:

  <!-- Images with explicit dimensions -->
  <img src="blog-image.jpg" width="800" height="450" alt="Article featured image">
  
  <!-- Font optimization with size adjustments -->
  <style>
    @font-face {
      font-family: 'CustomFont';
      src: url('/fonts/custom-font.woff2') format('woff2');
      font-display: swap;
    }
    
    @font-face {
      font-family: 'CustomFontFallback';
      src: local('Arial');
      size-adjust: 105%;
      ascent-override: 90%;
      descent-override: 25%;
    }
  </style>
  

• Result:

  • CLS score improved from 0.25 to 0.05, eliminating instances where readers lost their place or accidentally clicked ads instead of navigation elements.

Connections:

• Related Concepts:
  • Largest Contentful Paint (LCP): Optimizing for CLS often impacts how the largest elements load and appear.
  • Web Fonts Loading Strategies: Font loading directly impacts layout stability and CLS scores.
• Broader Concepts:
  • Web Vitals: CLS is one of the three Core Web Vitals that Google uses for ranking.
  • Responsive Web Design: Proper responsive techniques help maintain layout stability across viewports.

References:

1. Primary Source:
   • Web.dev CLS documentation: https://web.dev/articles/cls
2. Additional Resources:
   • Sentry's web vitals visualization tools
   • Browser DevTools Performance panel for debugging layout shifts

Tags:

#web-performance #user-experience #layout #core-web-vitals #frontend #visual-stability #optimization

Connections:

Sources:

• From: Syntax - Explicación de Web Vitals