Lighthouse

Lighthouse runs in Chrome DevTools, from the command line, as a Node module, in CI pipelines through Lighthouse CI, and behind PageSpeed Insights. It loads the page under controlled lab conditions, by default a simulated mid-range phone on a slow mobile connection, scores each category from 0 to 100, and lists audits such as oversized images, render-blocking scripts or missing alt text, each with an explanation.

Lab scores vary between runs and machines, and they are not what Google uses in search; that is field data from real Chrome users (Core Web Vitals). Treat the score as a diagnostic rather than a goal. Recent versions add an Agentic Browsing category that checks how easily AI agents can read and use a page, including a check for llms.txt.