Glossary of Web Platform Terms

Each entry's first mention is its <dfn> — the defining instance — with an id so other entries can cross-reference it.

A

ARIA: A set of attributes that supplement HTML semantics for assistive technology. The first rule of ARIA: prefer native elements — a <button> over role="button".
accessibility tree: The browser's parallel representation of the page that screen readers consume, built from element semantics, labels, and ARIA attributes rather than visual layout.

Baseline: A support status: a feature available in the current and previous versions of every major browser. Features outside Baseline need fallbacks or feature detection.
bidirectional text (bidi): Text mixing left-to-right and right-to-left scripts. HTML controls it with the dir attribute and the <bdi> element, which isolates runs of unknown direction.

content model: The specification of what children each element permits — flow, phrasing, sectioning, and so on. Violating the content model (a list item outside a list, block content inside a paragraph) makes a document invalid.
CDN: Geographically distributed servers that cache content close to users, cutting latency. Complements — never replaces — small, valid markup.
canonical URL: The authoritative address of a page, declared with <link rel="canonical"> so search engines fold duplicate routes into one ranking signal.