Glossary.

Google’s three-metric snapshot of user experience: loading speed (LCP), interactivity (INP), and visual stability (CLS).

A score for how much the page jumps around as it loads. Anything above 0.1 means visible jank.

When the browser first paints any text, image, or non-blank content. Usually the first signal that the page is alive.

How long it takes the page to visually respond to a user input. Replaced FID as a Core Web Vital in 2024.

How long the page takes to draw its biggest above-the-fold element. The headline metric in Google’s Core Web Vitals.

A score for how quickly a page’s content visually populates. Lighthouse-only, but a useful proxy for "felt" speed.

The total time the main thread is too busy to respond to user input — measured by Lighthouse, not by real users.

The official URL for a page when the same content is reachable at multiple addresses. Tells search engines which one to index.

The image that appears when a URL is shared on Twitter, LinkedIn, Slack, iMessage, etc. Specified via og:image meta.

The file at /robots.txt that tells crawlers which URLs they may and may not request. Convention, not enforcement.

A machine-readable index of every URL on the site. Hands search-engine crawlers a complete inventory without making them guess.

Machine-readable annotations in your HTML that tell search engines what the page is about. Powers rich results in SERP.

A content management system that exposes content via API instead of rendering its own pages. Decouples editing from presentation.

The process of attaching client-side JavaScript behavior to server-rendered HTML so the page becomes interactive.

An architectural pattern: client-side JavaScript, third-party APIs, prebuilt Markup. Optimizes for speed, scale, and security.

React components that run on the server and stream their output to the browser without shipping their JavaScript.

A browser API for declaring which links to prefetch or prerender, so navigation feels instant on hover.

Pre-rendering pages to static HTML at build time so the server has nothing to compute on each request. The fastest deploy model.

A browser-native way to animate between page states by snapshotting old and new DOM and crossfading the difference.

A set of HTML attributes that fill in semantic gaps for assistive tech. Used wisely it helps; used carelessly it hurts.

A CSS media query that respects users who’ve asked their OS to minimize motion. Vestibular disorders, migraines, focus.

The W3C standard for making web content accessible. Three conformance levels (A, AA, AAA); AA is the practical bar.

Running two versions of a page in parallel and measuring which performs better on a defined metric. The basis of empirical CRO.

A pricing model where a studio quotes a fixed price for a defined deliverable. Punishes both sides when scope shifts.

A monthly engagement where a studio acts as the ongoing steward of a website. Replaces the build-and-disappear project model.

A serverless function that runs at the CDN edge instead of a centralized region. Lower latency, smaller runtime.