# Lighthouse audits explained

Plain-English deep-dives on the Lighthouse audits that fail most often, with concrete fixes and code you can copy. Every page covers one audit ID, what it measures, why Google cares, common causes, and prescriptive fixes, written for developers solving real problems, not SEO marketers.

## Why these pages exist

Google's Lighthouse audit produces ~100 distinct checks. The official documentation is technically correct but optimized for completeness, not for "I have 30 minutes to fix this before deploy." These pages strip the audits down to:

1. What the audit actually measures
2. What the official "Good" / "Needs improvement" / "Poor" thresholds are
3. The 3–5 fixes that resolve 90% of real-world failures, with code
4. How to verify the fix without re-running the entire Lighthouse suite

Each page is also mirrored as raw markdown at `/audits/<slug>.md` for AI agents and LLM-based search to consume cleanly.

## Performance audits

These directly affect Core Web Vitals and your Google ranking.

- [**Largest Contentful Paint (LCP)**](/audits/largest-contentful-paint), how fast the biggest visible element renders. Core Web Vital, target under 2.5 s.
- [**Cumulative Layout Shift (CLS)**](/audits/cumulative-layout-shift), unexpected layout movement. Core Web Vital, target under 0.1.
- [**Total Blocking Time (TBT)**](/audits/total-blocking-time), main-thread blocking during load. Predicts how janky your page feels.
- [**Reduce unused JavaScript**](/audits/reduce-unused-javascript), bytes shipped but never executed. The most common bundle-bloat killer.
- [**Eliminate render-blocking resources**](/audits/render-blocking-resources), CSS and synchronous scripts that delay first paint.

## Accessibility audits

These affect screen reader users and your Lighthouse Accessibility score.

- [**Document does not have a main landmark**](/audits/document-main-landmark), the page lacks a `<main>` element. Screen readers can't jump to primary content.
- [**Heading elements not in sequentially-descending order**](/audits/heading-order), headings skip levels (h1 → h3). Breaks the document outline.
- [**Background and foreground colors do not have sufficient contrast**](/audits/color-contrast), text fails WCAG AA contrast requirements.
- [**Image elements do not have alt attributes**](/audits/image-alt), missing `alt` on `<img>`. Breaks accessibility AND image SEO.

## SEO audits

These directly affect how Google indexes and ranks your page.

- [**Document does not have a meta description**](/audits/meta-description), missing `<meta name="description">`. Tanks SERP click-through rate.

## Coming soon

Pages in the pipeline (likely shipping this month):

- `preload-lcp-image`, preloading the LCP element specifically
- `unused-css-rules`, unused CSS bytes
- `uses-text-compression`, gzip/brotli at the server
- `uses-responsive-images`, `srcset` and image CDNs
- `efficient-animated-content`, replace GIFs with MP4/WebM
- `font-display`, `font-display: swap` and the FOUT/FOIT trade-off
- `tap-targets`, touch target sizing on mobile
- `link-text`, descriptive anchor text
- `is-on-https`, TLS upgrade paths
- `meta-viewport`, the mobile viewport tag

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## How to use this library

If you're staring at a failing Lighthouse audit, find the matching page above and follow its fix steps. Each page is self-contained, you don't need to read them in order.

If you want a tailored brief for your specific URL, paste it at [lighthouse-md.com](/), we'll generate a `CLAUDE.md` covering every failing audit on your page with ranked offenders and code-level fixes, ready to drop into your repo or open Claude Code with one click.