Google Lighthouse is the free, open-source audit built into Chrome and into Google's PageSpeed Insights. Point it at a page and a minute later you get five scores from 0 to 100: Performance, Accessibility, Best Practices, SEO and, since 2026, Agentic Browsing. Those numbers get pasted into board decks, agency proposals and vendor comparisons, so it helps to know what each one actually measures, and what it does not.
Here is a plain-language tour of every score, the page-speed metrics behind them, and how to read them without overreacting.
How to Read a Lighthouse Score
Every category uses the same three color bands:
| Score | Band | What it means |
|---|---|---|
| 90 to 100 | Good (green) | The page passes nearly every check in that category. |
| 50 to 89 | Needs improvement (orange) | Worth a look. The report lists which checks failed and what fixing each would save. |
| 0 to 49 | Poor (red) | Something significant is wrong, usually a slow page or a broken basic. |
Lighthouse tests one page at a time, either on a simulated mobile phone (a mid-range device on a slower connection) or on a desktop. The same page often scores noticeably lower on mobile, and that is expected.
Performance: How Fast the Page Feels
The Performance score is a weighted blend of lab measurements taken while the page loads:
- Largest Contentful Paint (LCP): when the biggest piece of content, usually a hero image or headline, appears. Good is 2.5 seconds or less.
- Total Blocking Time (TBT): how long scripts keep the page from responding to a tap or click. Good is 200 milliseconds or less.
- Cumulative Layout Shift (CLS): how much the layout jumps around while it loads. Good is 0.1 or less.
- First Contentful Paint (FCP) and Speed Index: how quickly anything appears, and how quickly the visible area fills in.
LCP and CLS are two of Google's Core Web Vitals, which feed into search ranking. The third, Interaction to Next Paint, needs a real visitor clicking around, so Lighthouse uses Total Blocking Time as its stand-in. Large images, render-blocking scripts and heavy third-party tags are the usual causes of a low score.
Accessibility: A Useful Signal, Not a Compliance Grade
Lighthouse's Accessibility score runs a weighted subset of axe-core rules against one page: color contrast, missing alt text, form labels, button names, heading order and similar machine-checkable items. A low score is a reliable sign of real barriers.
A high score is not proof of the opposite. Automated rules can only judge roughly 30 to 40 percent of WCAG issues, a score of 100 covers a single page, and Lighthouse itself lists additional items for you to check by hand. A site can score 100 and still have a keyboard trap, unusable custom menus, or alt text that says nothing useful. Our guide to automated and manual testing explains where that line falls.
A score of 100 is not WCAG conformance
Treat the Lighthouse Accessibility score as a quick health check for one page. WCAG 2.1 AA conformance is judged across the whole site, against every success criterion, and needs manual testing for the parts no automated tool can evaluate.
Best Practices: Security and Modern Standards
This category looks at the plumbing: whether the page is served over HTTPS, whether the browser logged errors, whether images are displayed at a sensible size and aspect ratio, and whether the page relies on deprecated browser features or sets third-party cookies. Most failures here are quick fixes for a developer.
SEO: Can Search Engines Read the Page?
The SEO score checks the basics a search engine needs: a title and meta description, a successful HTTP status, crawlable links with descriptive link text, a valid robots setup, and a page that works on mobile. A score of 100 means the fundamentals are in place. It says nothing about content quality, keywords or backlinks, which is where ranking is actually won. There is a lot of overlap with accessibility here, which we cover in how ADA compliance boosts your SEO.
Agentic Browsing: Is the Page Ready for AI Agents?
Agentic Browsing is the newest category, added to Lighthouse in 2026 and still marked experimental by Google. It asks whether AI agents and assistants that browse on a person's behalf can understand and operate the page. Its checks include:
- A well-formed accessibility tree. Agents read much of the same structure screen readers do, so accessible markup pays off twice.
- Layout stability. An agent clicking a button that jumps away mid-load fails the same way a person does.
- An llms.txt file that follows the recommendations: a plain-text summary of the site written for AI assistants.
- WebMCP checks for pages that expose tools directly to agents. Most sites do not, so these show as not applicable.
Expect this category to change as the standards settle. For now it is a good early signal, and much of what it rewards is work an accessible site has already done.
Why Your Scores Change From Run to Run
Performance in particular can move 5 to 10 points between runs with no change to the site, because it depends on server response time and network conditions at the moment of the test. Two habits keep you from chasing noise:
- Compare the same page, on the same device setting, over several months rather than reacting to one run.
- Look at the individual metrics and the listed opportunities, not just the headline number. A rising LCP tells you far more than a score that dipped from 92 to 88.
How Ardor Tracks Lighthouse Scores for You
Every Ardor Accessibility plan includes a Website Health Report. Each month, and whenever your site is rescanned, we run Google Lighthouse on your home page on both mobile and desktop and keep the results. The report shows:
- The latest five scores for mobile and desktop, with the change from the previous month.
- Core Web Vitals and page-speed measurements next to Google's "good" thresholds.
- A month-by-month history, so you see the trend instead of a single snapshot.
- Each Agentic Browsing check, and the highest-impact suggestions your web team can act on.
It sits in your client portal next to your compliance documents. The scores are Google's, and we keep them separate from our own accessibility measurements, which come from scanning every page with our widget blocked and then active. One tells you how your home page performs; the other is the record of your accessibility work.
Want to see where your site stands? Run Google's own PageSpeed Insights (opens in a new tab) on your home page, try our free accessibility scan, or request a demo to see the Website Health Report for your own site.