Website Performance

Website performance

Core Web Vitals, caching, images, fonts, network behaviour. Curated from the Website Specification — Performance (CC BY 4.0).

Required

• Hit Core Web Vitals at the 75th percentile: LCP <= 2.5s, INP <= 200ms, CLS <= 0.1.
• Serve images in modern formats (WebP, AVIF), sized for the viewport, with explicit dimensions.
• Set Cache-Control: immutable, max-age=31536000 for fingerprinted assets; short/no-cache for HTML.
• Compress text responses (brotli where supported, gzip elsewhere); do not recompress media.

Recommended

• Native lazy loading (loading="lazy") for off-screen images/iframes — never on the LCP element.
• Resource hints: preload the LCP image and critical fonts, preconnect to third-party origins.
• Conditional requests: send ETag / Last-Modified, honour If-None-Match / If-Modified-Since.
• Self-host subsetted WOFF2 fonts with font-display: swap.
• Inline critical CSS; defer the rest.
• Choose the right <script> attribute: defer for app code, async for independent third-party, type="module" for modern code.
• Serve over HTTP/2 at minimum, HTTP/3 where possible.

Avoid

• HTTP/1.1 workarounds under HTTP/2+: domain sharding and image sprites.

Example

<link rel="preload" as="image" href="/hero.avif" />
<img src="/below-fold.avif" width="800" height="600" loading="lazy" alt="..." />

Full item list and rationale: https://specification.website/checklist/