Halfway down my Astro Rocket page there’s a section called “A perfect Lighthouse score,” with four numbers under it. All four are 100. This post explains what those numbers actually are, where they come from, how to check any website yourself in thirty seconds — and why they matter for a website you’re thinking of hiring me to build.
What is a Lighthouse score?
Lighthouse is an automated website auditing tool built by Google. It loads a page in a controlled environment, simulates a real visitor, and then grades the page on four separate dimensions — each scored from 0 to 100:
- Performance — how fast the page loads and becomes usable
- Accessibility — how well the page works for visitors with disabilities or assistive technology
- Best Practices — how cleanly and securely the page is built
- SEO — how easily search engines can read and index the page
The tool ships inside Chrome’s DevTools, but you don’t need to install anything to run it. Google hosts a free public version at pagespeed.web.dev — paste any URL and you get a full report in about thirty seconds, with both a mobile and a desktop score for that page.
Why four scores instead of one
A site can be lightning-fast and still be unusable to a screen reader. It can be perfectly accessible and still leak mixed-content warnings. It can pass every best-practice check and still be invisible to Google because the meta description is missing. The four categories exist because “good website” isn’t a single thing — it’s a stack of separate concerns, and Lighthouse measures each one independently so you can see which part is dragging the rest down.
Performance
Performance is the heaviest category and the hardest to keep at 100. It’s calculated from a handful of real, measured timings: how long until the visitor sees something on screen, how long until they can interact with it, and how stable the layout is while it loads (so a button doesn’t shift right as you go to tap it). These are the same Core Web Vitals that Google uses as a ranking signal in search.
A page that ships a megabyte of JavaScript will not score 100. A page that loads correctly-sized images, sends only the JavaScript it actually needs, and doesn’t block rendering on third-party scripts has a chance.
Accessibility
Accessibility is scored against a checklist of automated WCAG checks: text contrast, alt attributes on images, keyboard focus states, ARIA labels on icon-only buttons, semantic landmarks like <main> and <nav>. The audit cannot catch every accessibility issue — some checks genuinely require a human — but if a site fails the automated ones, it almost certainly fails the manual ones too. A 100 here is the floor, not the ceiling.
Best Practices
Best Practices catches the things you really should not be doing in 2026: serving pages over HTTP instead of HTTPS, using deprecated browser APIs, leaking errors to the console, embedding third-party iframes without the right sandbox attributes. It’s the “is anything obviously wrong?” category, and it’s the easiest of the four to keep at 100 if you start clean.
SEO
SEO in Lighthouse is the technical layer of search optimisation, not the content layer — it can’t tell you whether your post will rank, but it can tell you whether Google will be able to read it at all. The audit checks for a <title>, a meta description, a sensible canonical URL, indexable robots directives, valid hreflang if you’re multilingual, tap targets that aren’t crammed too close together on mobile, and so on. Get those right and you have a foundation. Skip them and even great content struggles.
How to run a report yourself
Three options, in increasing order of friction:
- pagespeed.web.dev — paste a URL, hit Analyse, read the report. No install, runs against Google’s servers, gives you both mobile and desktop in one go. This is the easiest way to verify any number I’m about to quote.
- Chrome DevTools — open any page, press F12, click the Lighthouse tab, choose Mobile or Desktop, click Analyse. Runs locally in your browser.
- The Lighthouse CLI —
npx lighthouse https://example.com --viewif you want to script it into a CI pipeline.
All three use the same audit engine. The score should match within a point or two between runs, with small variation coming from network conditions and CPU throttling.
What my own sites score
This site — hansmartens.dev — currently scores 100 / 100 / 100 / 100 on both desktop and mobile. Performance, Accessibility, Best Practices, SEO — perfect on every device profile, with no category dropping a point. That includes the simulated CPU throttling Lighthouse applies to mid-range mobile devices, which is the run that usually catches sites out.
For a visitor, that means a few specific things in practice:
- The first paragraph of any page is on screen well under a second on a normal connection.
- The page is fully readable and tappable before any animation, font swap, or third-party widget has a chance to interfere.
- A screen reader can navigate the site without hitting an unlabelled button, an empty link, or a missing heading level.
- Search engines see a complete
<title>, a real meta description, structured data, and a canonical URL on every page.
The score isn’t a tuned benchmark page either. It’s the live site, with the hero marquee, the dark/light theme switcher, the LetterGlitch animation in the desktop call-to-action, the scroll-progress ring, the cursor trail on desktop, the share buttons on every post — everything you can interact with is included in the audit.
And it isn’t a one-site trick. astrorocket.dev — the live demo of Astro Rocket, my free, open-source Astro theme — clears the same bar: 100 / 100 / 100 / 100 on both desktop and mobile. That demo is a complete website built from the theme’s component library, not a stripped-down landing page, so anyone who builds on Astro Rocket starts at 100 the moment they deploy.
Why I treat 100 as a constraint
Every new feature on this site goes through one extra check before it ships: what does it cost in Lighthouse? Most features cost nothing. Some cost a point or two. A few cost more, and those are the ones I either rebuild differently or leave out entirely.
The LetterGlitch animation in the call-to-action is the clearest example. On desktop it runs and looks great. On phones, the canvas animation would cost real Performance points on the Lighthouse mobile run — so there it doesn’t render at all, and a simple static call-to-action takes its place. The visitor still gets a polished CTA — they just don’t get a frame-rate-heavy canvas effect on a phone where the cost outweighs the benefit. That’s the kind of trade-off the score forces you to think about, and it’s the reason the mobile Performance number stays at 100.
Why your run might look slightly different
If you run the report yourself and see 98 instead of 100 on mobile Performance, that’s not a contradiction — it’s how the tool works. Lighthouse simulates a mid-range phone with throttled CPU and network on top of a real cold-cache page load, and small run-to-run variation is part of the design. Performance typically moves by one to three points between runs depending on Google’s lab load, network jitter, and whether the page is being fetched cold or warm. Accessibility, Best Practices, and SEO are deterministic checks and basically never move.
Two practical notes if your numbers look off:
- Use pagespeed.web.dev, not your local Chrome. Running Lighthouse in your own browser with ad blockers, password managers, or React DevTools installed can tank Performance by ten or more points — the extensions inject scripts into every page they audit. pagespeed.web.dev runs in a clean lab environment, which is the apples-to-apples comparison.
- Run it twice. The first run is always cold-cache; a second run a moment later usually settles on the steady-state number. If both runs land within a couple of points of each other on Performance, that’s the real score.
The numbers I quote in this post are the ones both live sites consistently produce on pagespeed.web.dev. Your run may land a point or two off on mobile Performance from time to time — that’s expected, and it doesn’t change the picture.
The bottom line
A perfect Lighthouse score isn’t a trophy — it’s a signal. It tells you the site is fast, works on assistive tech, is built cleanly, and can be found by search engines. And because I hold that bar on my own sites, it’s a promise I can make about yours: every website I build is held to it from the first commit, not patched towards it at the end.
Curious where your own website stands? Paste your address into pagespeed.web.dev and give it thirty seconds. If the numbers come back orange and red, that’s exactly what my free website check is for — send me your address via the contact form and I’ll show you what your site needs and what a move to Astro would change, with a timeline and a fixed price.