Glossary of site quality terms

A set of HTML attributes (role, aria-*) describing the role, state and name of UI elements for assistive technologies such as screen readers.

The address designated as the primary version of a page among duplicates (rel="canonical"). It consolidates ranking signals and prevents copies from being indexed.

An HTTP header declaring which sources the browser may load scripts, styles and other resources from. The main defense layer against XSS attacks.

The luminance ratio between text and its background, from 1:1 to 21:1. WCAG AA requires at least 4.5:1 for normal text and 3:1 for large text and UI elements.

A set of three Google metrics (LCP, INP, CLS) measuring loading speed, interaction responsiveness and visual stability of a page under real-world conditions.

The number of URLs a search engine bot can and wants to crawl on a site within a given time. Critical for large and frequently changing websites.

A Core Web Vitals metric measuring visual stability — the sum of unexpected layout shifts on a page. A good score is ≤ 0.1.

An HTTP header forcing the browser to connect to a site exclusively over HTTPS for a declared period. Protects against SSL stripping attacks.

A Core Web Vitals metric measuring the latency between a user interaction (click, tap, key press) and the next screen paint. A good score is ≤ 200 ms.

A Core Web Vitals metric measuring the time from page load start until the largest content element is rendered. A good score is ≤ 2.5 s.

A directive telling search engine bots not to add a page to the index. It does not block crawling — only indexing. Usually set via meta robots or the X-Robots-Tag header.

Measuring performance and errors from data collected from a site's real users — in their browsers, on their networks and devices.

A standardized format for describing page content (usually JSON-LD with the Schema.org vocabulary) that helps search engines understand a page and qualify it for rich results.

The time from sending a request until the first byte of the server response arrives. The foundation of a fast LCP; a good score is roughly ≤ 800 ms.

Uptime is the percentage of time a service is operational; an SLA is a contractual guarantee of that level with consequences for breaching it. "Three nines" = 99.9% = ~8.7 h of downtime per year.

The W3C international standard for web content accessibility. It defines success criteria at levels A, AA and AAA; the current version is WCAG 2.2.

The CO₂e emissions generated by a website: data transfer, data centers, networks and user devices. Estimated with models such as Sustainable Web Design v4.

An XML file listing a site's URLs with metadata (lastmod) that helps search engines discover and refresh pages. Defined by the sitemaps.org standard.