Google Lighthouse

Lighthouse is a free, open-source tool designed to conduct comprehensive technical audits of websites. Developed by Google, it evaluates various aspects of a URL, including performance, accessibility, best practices, SEO, and Progressive Web Apps.

This framework is integrated into Google's other performance analysis tools like PageSpeed Insights and Chrome's developer tools for browser-based audits.

What is Google Lighthouse?

Lighthouse enables website performance audits. Initially, Lighthouse 1.0 was created to audit Progressive Web Apps (PWAs). Version 2.0 expanded to include performance and SEO analyses for regular websites. By early 2018, Lighthouse 3.0 introduced a new layout and direct integration into Chrome's developer tools. It continues as an open-source analysis framework functioning at the URL level.

Which audits can Google Lighthouse run?

Lighthouse audits are categorized into five optimization areas: Performance, Best Practices, Accessibility, SEO, and Progressive Web Apps.

Users can choose any combination of these categories to analyze specific aspects of their website.

Performance

This category evaluates the loading speed and accessibility of website content. Lighthouse assesses six key speed metrics:

• First Contentful Paint: Time until users see the first text or image.
• First Meaningful Paint: Time until the main page content is visible.
• Speed Index: A metric indicating how fast the page content is loaded.
• Time to Interactive: Time until users can fully interact with the page.
• First CPU Idle: Time until the main thread is idle enough to process user inputs.
• Estimated Input Latency: Estimated time (in milliseconds) for the app to respond to user inputs during peak load times. Latency over 50ms may cause the site to feel sluggish.

Lighthouse provides performance improvement suggestions like compressing images, optimizing JavaScript and CSS, and reducing render-blocking resources. Recommendations also include effective caching, minimizing server response times, and reducing redirects.

Performance recommendations from Lighthouse include:

• Reduce render-blocking resources
• Use next-gen image formats
• Enable text compression
• Defer unused CSS
• Ensure text visibility during font loading
• Implement efficient cache policies
• Enhance the Critical Rendering Path (CRP)
• Avoid oversized images
• Delay loading offscreen images
• Compress CSS and JavaScript
• Optimize images
• Use server pre-connect
• Keep server response times low (TTFB)
• Avoid redirects
• Preload key requests
• Utilize video formats for animations
• Reduce total byte weight
• Limit excessive DOM size
• Use user timing marks for performance measurement
• Minimize JavaScript bootup time
• Reduce main thread payload

Best Practices

Lighthouse evaluates 16 Best Practices, focusing on security and modern web development standards. It checks for HTTPS and HTTP/2 usage, secure resource sourcing, and JavaScript library vulnerabilities. Additional practices include secure database connections and avoiding outdated commands like document.write() and obsolete APIs.

SEO

Lighthouse conducts tests to assess a website's crawlability and search engine display efficiency. These SEO tests are limited, but any site not achieving a perfect score should address the issues identified. Effective search engine optimization can significantly enhance visibility and performance.

Lighthouse runs 13 SEO audits focusing on mobile-friendliness, PWAs, structured data application, tags like canonicals and hreflang, and crawlability by search engine bots.

Accessibility

Lighthouse's accessibility audits check how well a website or app accommodates users with disabilities. Tests include evaluating button and link descriptions and ensuring images have alt attributes for screen readers to describe visual content to visually impaired users.

Progressive Web Apps

Initially the core focus of Lighthouse, Progressive Web App analysis examines service worker registration, offline functionality, and error handling. Although now one of five audit categories, it remains crucial for PWA providers.

How can Google Lighthouse be accessed?

• PageSpeed Insights: This tool now incorporates Lighthouse analysis, offering URL testing for six performance metrics and additional audit results and recommendations. Access PageSpeed Insights here.
• Developer Tools in Chrome: Available in Chrome's developer tools since April 2018, users can select audit categories, device options, and simulated internet speeds for analysis. Learn how to locate Lighthouse audits in Chrome DevTools here.
• Chrome Plugin: Run complete Lighthouse audits using the Chrome Plugin. Find it here for Chrome users.
• Lighthouse's Performance Audits are also accessible via Google’s web.dev portal. Discover key audits and recommendations here.

How can I audit a complete website?

Lighthouse is an effective tool for assessing website and PWA performance. However, its primary limitation is running audits at the URL level.

Conductor Website Monitoring has developed a method using Lighthouse Audits to analyze entire domains and their online markets, determining optimization levels within an industry and comparing domain performance against competitors.