C
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
What is CLS (Cumulative Layout Shift)?
CLS measures how much content shifts unexpectedly while a page is loading or in use, and Google counts anything under 0.1 as good.
The calculation has two factors: how much of the screen the shifting area covers and how far it moves. That is why a small shift of a large element at the top is penalised more than a large shift of a small element further down.
Only UNEXPECTED shifts are counted. Shifts occurring within half a second of a user interaction are exempt — which is what makes accordions and dropdowns free.
There are four common sources: images without dimensions, font swapping, content inserted later, and embedded frames without dimensions. The first is the most common and the cheapest to fix — writing a width and a height on the image.
Total shift is not summed across the whole life of the page; shifts are divided into session windows and the reported value is that of the WORST window. The problem, in other words, is not the total but the concentration.
The measured value for this site and the three projects it manages is 0.001 — one hundredth of the good threshold. The verification suite ties the threshold to 0.05; leaving room below the official threshold is deliberate, so that a regression shows before it hits the limit.
The most overlooked source lies where classic measurement cannot see: shifts occurring during scrolling. Auditing tools look at the page load window and do not scroll; the user, meanwhile, experiences exactly those shifts.
On this site that blind spot was found by measurement and closed with its own guard: the worst window computed with session-window logic across the whole scroll has to stay under 0.08.
- Write a width and a height on images — a one-line fix that closes the most common source.
- Match the fallback font metrically — so there is no shift at the moment of the swap.
- Do not push later content into the flow — show it as a layer or reserve its space from the start.
- Give embedded frames a ratio — video, maps, social boxes.
- Animate only with transform and opacity — layout properties trigger a reflow on every frame.
- Test on a throttled network — on a fast connection the shifts are invisible.
The last item is a precondition for measuring at all: on a fast machine everything arrives almost simultaneously, so the shifts never appear.
Knowing that the measurement window works on session logic matters too: the reported value is not the total but that of the worst window formed by shifts close together. The problem is not how many shifts there are but that they gather in one place.
This measure is not read on its own; the set in which three metrics are assessed together is Core Web Vitals.
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
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
Core Web Vitals is the collective name for the three metrics Google defined to measure a page’s real user experience. 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