Glossary

Answer engine optimisation: the work of making pages easy for AI assistants to find, read and cite. It builds on SEO, because assistants mostly answer from a live web search.

A short text alternative for an image, written in its `alt` attribute. Screen readers read it aloud and search engines use it; purely decorative images get an empty one.

An image format based on the AV1 video codec. At the same visual quality its files are usually clearly smaller than JPEG and WebP, and all current major browsers display it.

The part of a website or app that runs on a server. It stores data and computes answers that depend on the person using it, such as bookings, logins or prices.

A lossless compression format developed at Google and specified in RFC 7932. For text files such as HTML, CSS and JavaScript it usually produces smaller files than gzip.

The automated step that turns source files, texts and images into the finished files of a website. For a static site it produces every page in advance.

Keeping a copy of a response so it does not have to be fetched or computed again. The Cache-Control header tells browsers and CDNs how long a copy stays valid.

The preferred address of a page when several URLs show the same content. The element `rel="canonical"` tells search engines which one to index and display.

Content delivery network: servers in many locations that keep copies of a website's files and deliver them from a location near the visitor, which shortens the distance and the waiting time.

Cumulative Layout Shift: how much visible content moves unexpectedly while a page loads and is used. Google rates a score of 0.1 or less as good.

Content management system: software with an editing interface in which texts, images and pages change without writing code. WordPress runs 35.6 percent of all mobile pages (Web Almanac 2025).

Encoding a file so that it needs fewer bytes in transit, after which the browser decodes it. The server names the method in the Content-Encoding header, usually gzip, Brotli or Zstandard.

The luminance ratio between text and its background, from 1 to 1 up to 21 to 1. WCAG 2.2 requires at least 4.5 to 1 for normal text and 3 to 1 for large text.

Google's three measures of page experience: LCP for loading, INP for responsiveness and CLS for visual stability. Each is judged at the 75th percentile of real visits.

A program that loads web pages automatically, follows their links and passes the content to a search index or a training data set. Each identifies itself by name, such as Googlebot or OAI-SearchBot.

Publishing a new version of a website by putting its built files on the server or CDN, where visitors can reach them.

The long-established compression format of the web, specified in RFC 1952. Every browser understands it, so it remains the fallback when a browser offers nothing newer.

An annotation that links the language versions of a page to each other, so that search engines show each searcher the version in their language.

Interaction to Next Paint: how quickly a page visibly responds to clicks, taps and key presses during a visit. Google rates 200 milliseconds or less as good.

An image format standardised as ISO/IEC 18181, clearly smaller than JPEG, that can also repack existing JPEG files losslessly. Chrome from version 155 and Firefox from 158 support it, Safari only in part, so a fallback format remains necessary.

Postponing an image, video or embed until the reader scrolls near it. Browsers do this natively with the attribute `loading="lazy"`.

Largest Contentful Paint: the time until the largest image or text block in the first screen is rendered. Google rates 2.5 seconds or less as good.

A rule in the HTML or in an HTTP header that keeps a page out of search results. It stops indexing, which a robots.txt rule cannot.

An instruction in the HTML (`<link rel="preload">`) that tells the browser to fetch a file early because the page will need it soon, typically a font or the main image.

Search results with extra elements such as images, ratings or prices, built from structured data. Google decides when to show them and supports only certain types; FAQ rich results ended on 7 May 2026.

A text file at the root of a website that tells crawlers, one by one by name, which paths they may load. It controls crawling, not whether a page appears in search results.

Software that reads the content of a screen aloud or sends it to a braille display. It depends on headings, labels and text alternatives in the code.

Search engine optimisation: the work of making pages easy for search engines to crawl, index and choose as a result for relevant queries.

An XML file that lists the addresses of a website that should be indexed. It helps search engines find new and changed pages sooner.

A website whose pages are generated as finished HTML files before anyone visits. The server only delivers files and computes nothing per request.

Machine-readable facts about a page, usually schema.org vocabulary in JSON-LD. They can make a page eligible for rich results and must match what the page visibly says.

A font file reduced to the characters a website actually uses, for example only Latin letters. Removing unused alphabets and symbols makes the file noticeably smaller.

Time to First Byte: the time from the start of a navigation until the first byte of the response arrives. web.dev rates 0.8 seconds or less as good and more than 1.8 seconds as poor.

A font file that contains a continuous range of styles, for example every weight from thin to black, instead of one file per style. A page with several weights then loads one file.

Web Content Accessibility Guidelines: the W3C standard for accessible web content, currently version 2.2. Laws and public standards usually require its level AA.

An image format developed by Google, lossy or lossless, with transparency and animation. It is clearly smaller than unoptimised JPEG but only slightly smaller than a well-optimised one.

The W3C font format for the web. It compresses font data with Brotli, so a WOFF2 file is considerably smaller than the same font as TTF or OTF.

A lossless compression format developed at Meta (then Facebook) and specified in RFC 8878. The browser announces in its Accept-Encoding header whether it understands zstd, so a server can fall back to Brotli or gzip.