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Lighthouse performance scoring

Kenny DuMez
Kenny DuMez
Graphite software engineer

Google Lighthouse is an open-source, automated tool for improving the quality of web pages. It provides a set of audits against a web page, covering areas such as performance, accessibility, progressive web apps, SEO, and best practices. Each category is scored from 0 to 100, reflecting the health and usability of your site. This guide will explore the various aspects of Lighthouse scores, their significance, and how to interpret and improve these metrics.

A Lighthouse score is a numerical measure provided by Google's Lighthouse tool, which evaluates various aspects of a web page's quality. Each main audit category—Performance, Accessibility, Best Practices, SEO, and Progressive Web Apps—receives its own score. These scores help developers understand how well their site performs on a range of metrics and where there is room for improvement.

  1. Performance score: Measures how well the page performs on metrics such as load time, interactivity, and the stability of content as it loads. This score is crucial for understanding user experience, particularly on mobile devices.
  2. Accessibility score: Assesses the page's accessibility for users with disabilities. This includes checks for screen reader support, keyboard accessibility, and the use of ARIA roles and attributes.
  3. Best practices score: Evaluates the page against best practices in web development, covering everything from HTTPS usage to correct image aspect ratios.
  4. SEO score: Measures how well the page is optimized for search engine visibility, including meta information, HTTP status codes, font sizes, and more.
  5. Progressive web app (PWA) score: Checks how well the page adheres to the principles of a Progressive Web App, including its responsiveness, offline capabilities, and compliance with modern web standards.

Each Lighthouse score is calculated based on a series of audits. Lighthouse runs these audits by loading the page in a controlled environment (a simulated mobile device by default) and examining various elements and performance metrics. Once these aspects of the site are evaluated, Lighthouse assigns a score using the following scale:

  • 0-49 (poor): Indicates significant room for improvement.
  • 50-89 (average): Suggests that the page is on the right track but could be enhanced.
  • 90-100 (good): Represents high performance and adherence to best practices.

Improving your Lighthouse scores involves addressing the specific issues highlighted in the Lighthouse report. Here’s how you can approach improving scores in each category:

  • Optimize images: Use compression tools to reduce the size of images without losing quality.
  • Minimize JavaScript and CSS: Remove unnecessary code, minify scripts and stylesheets, and defer the loading of non-critical resources.
  • Improve server response times: Optimize your server, use a Content Delivery Network (CDN), and cache content effectively.
  • Semantic HTML: Use proper HTML elements for content (e.g., article, section, nav).
  • Alt text for images: Ensure all images have descriptive alternative text.
  • Keyboard navigability: Make sure all interactive elements are accessible via keyboard.
  • Use HTTPS: Secure your site with HTTPS to protect user data.
  • Avoid deprecated APIs: Replace outdated APIs with modern alternatives.
  • Responsive images: Use appropriate sizes attributes and srcset to ensure images are responsive.
  • Meta tags: Use descriptive titles and descriptions.
  • Structured data: Implement markup to enhance search engine results.
  • Accessible content: Ensure content is readable and navigable.
  • Service workers: Implement service workers for offline support.
  • Web app manifest: Include a manifest file with metadata about the app.
  • Responsive design: Ensure the app is usable on any device size.

For further reading see Google's official Lighthouse documentation.

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