WTF is a Lighthouse score?

We break it down.

WTF is Lighthouse, anyway?
Lighthouse is an open-source tool integrated into Google’s Chrome browser that allows web developers to evaluate the performance of their sites across several different metrics, including page speed, SEO, accessibility (as it relates to visitors with disabilities), the page’s functionality as a progressive web app, and a list of best practices compiled by a community of developers. Each of the five metrics is rolled up into an overall score, which can range from 0 to 100.

How is it different from other page performance tools?
There’s no shortage of tools, both free and paid, that developers can use to evaluate those things. But most other performance tools tend to specialize in one or two things. Instead of using multiple tools to evaluate a site’s speed and its accessibility, Lighthouse allows users to check everything in a single interface, for free.

But does that make it better?
Depends on how you look at it. While some tools will consider more factors when evaluating a site — Lighthouse only uses five things to provide a site speed score, for example — Lighthouse offers an ease of use that most other tools don’t. A developer can check a page’s score inside Chrome’s browser using DevTools or a Chrome Extension, from the command line on their own server, or as a Node tool, which makes it easy to integrate into almost any workflow or move.

Does having a good Lighthouse score guarantee that a site will perform well in a Google search?
It probably doesn’t hurt. As with many other Google products, the scores are a bit of a black box. At minimum, they help developers make their sites load more quickly, which helps.

Why are people so obsessed with site speed right now?
Because site speed affects search results. Speed has played a role in desktop search results for almost a decade, but Google didn’t make site speed a factor in mobile search results until last summer. And with most sites now significantly more reliant on mobile visitors than desktop ones, speed is more important than ever. Perhaps not coincidentally, Lighthouse started becoming more of a topic of conversation around that time.

Should everybody be trying to get a perfect score?
Everybody likes to get a good grade on their work, but the realities of ad-tech make it pretty much impossible for an ad-supported publisher’s site to get a perfect score. For example, sites that use lots of Javascript to serve the ads on their pages may find themselves losing lots of points. Similarly, a developer that wants their site to run smoothly on the widest range of browsers might not want to use next-gen image formats, even though doing so can improve a Lighthouse score.

A site’s willingness to use Google’s tools affects a score too. If a publisher decides they don’t need their site to work as a progressive web app, for example, that will dent the PWA score that factors into any site’s overall Lighthouse score.