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Web and SEO terms made simple

I give clear definitions, map related terms, and link to reliable resources.
I update it as I learn.

About this glossary

Core Web Vitals – Glossary

Google’s user experience metrics for loading, interactivity, and visual stability.

  • LCP: Largest Contentful Paint, the time until the main content is visible.
  • CLS: Cumulative Layout Shift, unexpected movement of elements during load.
  • INP: Interaction to Next Paint, how long a page takes to respond to input and render the next frame.

Targets

  • LCP: 2.5 s or less (Good), 2.5–4.0 s (Needs improvement), over 4.0 s (Poor)
  • CLS: 0.1 or less (Good), 0.1–0.25 (Needs improvement), over 0.25 (Poor)
  • INP: 200 ms or less (Good), 200–500 ms (Needs improvement), over 500 ms (Poor)

How to measure

  • Field data (real users), use the Chrome UX Report and analytics to track performance over time.
  • Lab data (simulated), use Lighthouse or local runs to test repeatably and debug changes.
  • Prefer field data for decisions, use lab runs to validate fixes before shipping.

Common fixes

LCP (loading):

  • Serve the hero image in AVIF or WebP, set width and height, compress aggressively.
  • Inline critical CSS, defer non‑critical CSS and JS, preload the LCP resource if it is a hero image or key font.
  • Reduce server time to first byte, cache HTML for anonymous users, use a CDN close to your audience.

CLS (visual stability):

  • Reserve space with width and height or CSS aspect‑ratio for images, embeds, and ads.
  • Avoid injecting content above the fold after load, place banners and consent prompts below the main content.
  • Load fonts predictably, use font‑display settings that prevent large layout shifts.

INP (interactivity):

  • Keep the main thread light, split bundles, remove unused third‑party scripts, lazy load heavy UI only when needed.
  • Avoid long synchronous tasks, debounce expensive handlers, move work off the main thread with Web Workers.
  • Simplify frequent interactions, reduce DOM complexity, prefer lightweight components.

Checklist

  1. Confirm targets with field data, then reproduce with lab runs.
  2. Identify the LCP element, optimize the asset (format, size), and preload if needed.
  3. Fix layout shifts by reserving space for all media and UI.
  4. Profile interactions, eliminate long tasks, and trim third‑party scripts.
  5. Recheck in the field after shipping, aim for week‑over‑week improvements.

Further reading

Help

Web Vitals, Google

Overview of the program, definitions, lifecycle, and tools with thresholds for LCP, INP, CLS.

https://web.dev/vitals/

Help

Optimize LCP, web.dev

Step by step approach to improve LCP, covering TTFB, preload discovery, fetchpriority, render blocking CSS/JS, and CDN caching.

https://web.dev/optimize-lcp/

Help

Optimize CLS, web.dev

Prevent layout shifts by reserving space for media and embeds, stabilizing ads and banners, using aspect ratio, and managing font swaps.

https://web.dev/optimize-cls/

Help

Optimize INP, web.dev

Measure and improve responsiveness across interactions, break up long tasks, minimize main thread work, and use RUM to find slow interactions.

https://web.dev/inp/

Related terms

Lazy loading

Images and embeds that defer offscreen content to speed up pages.

Preload and preconnect

Hints that help the browser fetch critical files sooner and open connections early.

Third-party scripts

Measure impact, defer when possible, and remove tags you do not need.

Compression

Enable Gzip or Brotli so HTML, CSS, and JS transfer faster.

Category:
Category
Site health and performance
· Metrics and monitoring

Last updated:

About this glossary

I’m building this glossary to become a better developer and website strategist, in my ongoing quest to build clearer, better websites. It maps how terms relate, adds practical examples, and highlights best practices so I can communicate more clearly with clients. If it helps others too, great.

It’s a living resource, I aim to cite sources clearly. If you spot errors or omissions, please contact me so I can update it. Drafts may be assisted by AI, I review and fact check before publishing. For details on how I use AI, see AI Use at Webmarks.