Core Web Vitals SEO Rule

Analyze page elements that impact Core Web Vitals including render-blocking resources, image optimization, and third-party scripts

What This Rule Checks

Checks for render-blocking CSS (>3 files) and JavaScript (no async/defer), missing preload hints, large images affecting LCP, images without dimensions causing CLS, third-party scripts (>3), and missing viewport meta tag.

Why It Matters for SEO & GEO

Core Web Vitals (LCP, FID, CLS) are Google ranking factors. Poor performance leads to lower rankings, higher bounce rates, and worse user experience across all devices.

How to Fix

Add async/defer to scripts. Combine or inline critical CSS. Add width/height to images. Reduce third-party scripts. Use preload for critical resources. Optimize image sizes.

Examples

Bad

<script src="analytics.js"></script><script src="tracking.js"></script>

Good

<script src="analytics.js" defer></script><link rel="preload" href="critical.css" as="style">

How VibeLinter Checks Core Web Vitals Factors

VibeLinter’s performance/core-web-vitals rule performs these static analysis checks:

Render-blocking CSS — Warns when more than 3 non-print stylesheet links are found (affects LCP)
Render-blocking JavaScript — Warns about <script src> tags without async or defer attributes (affects FID/INP)
Missing preload hints — Suggests <link rel="preload"> when critical CSS exists without preload hints
Large images (LCP) — Detects images with pixel counts over 800,000 (e.g., 1000x800) that may affect LCP
Missing dimensions (CLS) — Counts images without width and height attributes that cause layout shifts
Third-party scripts — Warns when more than 3 third-party scripts are detected (Google Analytics, GTM, Facebook, DoubleClick, AdSense, Google Ads)
Missing viewport — Errors when viewport meta tag is absent, affecting mobile performance

Configuration

// vibelinter.config.cjs
module.exports = {
  rules: {
    'performance/core-web-vitals': {
      enabled: true,
      severity: 'warning'
    }
  }
}

SEO Impact

Core Web Vitals directly affect:

Search rankings — Google uses CWV as a ranking signal in the page experience system
User engagement — Fast pages have lower bounce rates and higher conversion rates
Mobile experience — Performance issues are amplified on mobile devices with slower connections
GEO (Generative Engine Optimization) — AI search engines factor in page quality signals including performance; fast-loading pages with good CWV scores are more likely to be selected as authoritative sources in AI-generated answers