Cache header A cache header is the server response header that tells the browser how long a resource may be stored and how it is to be revalidated. Performance

Canonical address A canonical address is the markup declaring which page is the original when the same or very similar content is reachable from more than one address. Multilingual

Citation A citation is an answer engine mentioning a source in the answer it produces, usually with a link to it. Answer engines

CLS (Cumulative Layout Shift) measures how much content shifts unexpectedly while a page is loading or in use, and Google counts anything under 0.1 as good. Performance

Colour blindness is the inability to tell certain colours apart, and it affects a considerable share of the world’s population, men in particular. Accessibility

Compression is the shrinking of text-based resources before they are sent from the server and their expansion in the browser. Performance

What are Core Web Vitals?

Core Web Vitals is the collective name for the three metrics Google defined to measure a page’s real user experience.

The three metrics measure three separate dimensions and none substitutes for another: loading speed, interaction responsiveness and visual stability. For a page to count as good, all three have to pass the threshold.

LCP — the paint time of the largest visible content. Good threshold: 2.5 seconds.
INP — the visual response time after an interaction. Good threshold: 200 milliseconds.
CLS — unexpected layout shift. Good threshold: 0.1.

All three are assessed in field data at the 75th percentile — the value experienced by three quarters of users. That keeps a single bad sample from spoiling the score while letting a widespread problem show.

The set of metrics changes over time: INP replaced FID in 2024, and new metrics may be added in future. So improving the experience a metric measures is a more durable approach than optimising for the metric itself.

The data used in ranking is field data, not a lab score. Getting a high score in an auditing tool does not mean being good in Core Web Vitals terms.

The three metrics are independent of one another: a page can open very quickly and respond late to clicks, or be fast and responsive and still shift under you while loading. Each has its own treatment, and the work that fixes LCP usually does not touch INP.

Its weight in ranking is limited, and Google says so plainly: content relevance always comes first. Poor Core Web Vitals do not bury a good page, but they can make the difference between two equivalent ones.

The real justification is not ranking but conversion: a slow, unstable page loses the user regardless of ranking.

If LCP is poor — look at resource size, discovery delay and render blockers.
If INP is poor — take an inventory of JavaScript, break up long tasks.
If CLS is poor — dimension reservation, font matching and content arriving later.
If all three are poor — look at the architectural decision; optimising one by one will not do.
If all three are good — the purpose of measurement is no longer improvement but guarding against regression.

The last item is a way of working: once the metrics fall below the threshold the job is not over, protection begins — and protection only works if it is tied to a test.

If you want your own site measured and fixed at the source, our Core Web Vitals optimization starts from real-user data and delivers LCP, INP and CLS against a budget.

Performance

Country domain (ccTLD) A country domain is a two-letter top-level domain extension allocated to a particular country. Multilingual

Crawl budget is the number of requests a search engine allocates to fetching from a site within a given period. Answer engines

Critical rendering path The critical rendering path is the minimum sequence of steps a browser must complete before it can draw a page on screen. Performance

E

Entity An entity means a distinct thing that search and answer engines recognise — a person, an organisation, a place, a product or a concept. Answer engines

F

Field data is the sum of performance measurements experienced by real users on real devices and real networks. Performance

Focus trap A focus trap is when keyboard focus enters a region and cannot leave it again, and WCAG prohibits it explicitly at level A. Accessibility

Font subset A font subset is a font file regenerated with only the characters that are needed. Multilingual

G

GEO (Generative Engine Optimization) is the work of increasing the likelihood that a site is chosen as a source in the answers generated by AI systems. Answer engines

H

hreflang is the markup that declares a page’s different language and region versions to search engines. Multilingual

I

IndexNow is an open protocol that tells search engines immediately that a page has changed. Answer engines

INP (Interaction to Next Paint) is the time from a user interaction until the page responds visually, and Google counts anything under 200 milliseconds as good. Performance

J

JSON-LD (JavaScript Object Notation for Linked Data) is a structured data format that declares a page’s meaning in a machine-readable form without changing the page’s appearance at all. Answer engines

L

Language tag (BCP 47) A language tag is the standard coding form that tells machines which language a text is in. Multilingual

LCP (Largest Contentful Paint) is the paint time of the largest content element in a page’s visible area, and Google counts anything under 2.5 seconds as good. Performance

Live region A live region is an area of a page whose changes a screen reader has to announce to the user. Accessibility

llms.txt is an optional file placed in a website’s root directory that summarises, in Markdown, what the site is about and where its most important pages are for language models. Answer engines

Locale A locale is the setting that defines a user’s language and region together and determines the formatting rules. Multilingual

Localisation (Localization) is adapting content to the culture, habits and search language of a target market; it is a wider undertaking than translation. Multilingual

R

Reflow A reflow is the browser recomputing the positions and sizes of elements on a page after a change. Performance

Relative luminance is the value expressing a colour’s perceived lightness between 0 and 1, and it is the basis of the contrast ratio calculation. Accessibility

S

Screen reader A screen reader is assistive technology that turns on-screen content into speech or braille output and lets the user navigate by keyboard. Accessibility

Semantic HTML means choosing each element for its meaning rather than its appearance — a heading element for a heading, a button element for a button, a list element for a list. Accessibility

Structured data is an additional layer that labels a page’s content with a standard vocabulary machines can understand. Answer engines

T

Text expansion is the same content being of different lengths in different languages, and the layout having to flex accordingly. Multilingual

Translation memory A translation memory is a database that stores previously translated sentences and suggests them when the same or a similar sentence comes up again. Multilingual

TTFB (Time to First Byte) is the time between the browser sending a request and receiving the first byte from the server. Performance

W

WCAG (Web Content Accessibility Guidelines) is the international standard published by the W3C that defines the accessibility of web content through testable success criteria. Accessibility

X

x-default is a special hreflang value declaring the default page to show when no language version matches the user’s language. Multilingual