Core Web Vitals

The part of a page visible without scrolling, a newspaper idea carried onto the web, where it still matters for first impressions and loading but no longer means people won't scroll.

Browsers keeping a whole page frozen in memory when you navigate away, so pressing Back restores it instantly, unless something on the page makes it ineligible.

The automatically-derived list of pages linking to the current one.

The sequence of steps between receiving HTML and painting pixels, and everything that blocks it.

An opaque fingerprint of a response's content that lets a client ask a server whether it's changed without re-downloading it.

How evenly frames are delivered over time, which shapes perceived smoothness more than average frame rate does.

When a measure becomes a target, it stops being a good measure.

The set of headers that let a browser or intermediary reuse a previous response instead of re-fetching it.

Two successive rewrites of HTTP's transport layer, each solving the previous version's head-of-line blocking at a different layer.

The file formats the web uses for pictures, from the GIF, JPEG, and PNG generation to newer formats derived from video codecs, and the slow, politics-laden process by which a new one becomes safe to use.

Rendering a page as static HTML with small independently interactive regions.

Visible content jumping position after it first appears, usually because something loaded late without reserved space, measured by browsers as Cumulative Layout Shift.

Deferring the fetch of a resource until it is needed, or until the browser is idle.

Any stretch of main-thread work over 50 milliseconds, during which the page can't respond to input, and the techniques for breaking such work up.

A committed numeric ceiling on a page's cost, enforced automatically.

Two resource hints with opposite priorities — preload for what this page needs now, prefetch for what the next page might need.

Spinners, progress bars, and step counters that tell people a system is working and how long is left, which changes how long a wait feels more than how long it is.

Offering the browser several versions of an image so it can download the smallest one that still looks sharp on this screen at this layout size.

Making content discoverable through search, most of which is ordinary quality work.

Producing a page's HTML on the server so the first response is already content.

A web app that loads one HTML page and then rewrites it with JavaScript as the user navigates, trading a slower, heavier first load for app-like transitions afterwards.

A gray outline of the layout shown while content loads, so the page feels faster and doesn't jump when the real content arrives.

The slowest few percent of response times, which averages hide and which dominate the experience of large systems, because one request often waits on many others.

Measuring how people use a website, from parsing server logs to JavaScript page tags and Google's free Analytics, with metrics like pageviews and bounce rate that are distorted by bots, blockers, and consent rules and easy to optimize in the wrong direction.

How custom typefaces are fetched and shown, and why the cost is easy to misattribute.