Glossary

Compact definitions of terms used across the reference cards. Each entry links to the longer treatment and related entries.

• Accessibility tree

  The parallel tree of nodes the user agent maintains alongside the DOM, exposing each element's name, role, state, value, and relationships to platform accessibility APIs and assistive technology.

• ARIA pattern

  A documented composite widget design — combobox, tabs, dialog, listbox, menubar, tree — with conventional roles, states, properties, and keyboard interactions defined by the WAI-ARIA Authoring Practices Guide.

• Assistive technology

  Software or hardware that mediates between the user and the operating system or a web page, translating between the platform's accessibility tree and a modality the user can perceive or operate.

• Cascade

  The CSS algorithm that resolves conflicting declarations into a single computed value per property per element. Considers origin, importance, layer order, specificity, and source order in that sequence.

• Cumulative Layout Shift (CLS)

  A Core Web Vital measuring the sum of unexpected layout shifts during the page lifecycle. The 75th-percentile target is 0.1 or less. Driven mostly by images without explicit dimensions and dynamically inserted DOM.

• Focus ring

  The visible indicator a user agent draws around the currently focused element, announcing keyboard or programmatic focus position. The ring is the only cue keyboard users have for tracking focus.

• Forced-colors mode

  An operating-system high-contrast setting that overrides author colours with a system palette. CSS exposes it via @media (forced-colors: active) and the system-colour keywords CanvasText, ButtonFace, Highlight.

• Largest Contentful Paint (LCP)

  A Core Web Vital measuring the render time of the largest above-the-fold element — usually a hero image, video poster, or headline block. Target: 2.5 seconds or less at the 75th percentile.

• Logical properties

  CSS properties keyed to the writing mode and direction (block-start, inline-end, padding-block) rather than the physical screen (top, right, padding-top). Adapt automatically to right-to-left and vertical scripts.

• Nominative fair use

  The legal doctrine that permits using another party's trademark to refer to the trademark holder's product, when the reference is necessary, no broader than necessary, and does not suggest endorsement.

• Service worker scope

  The URL prefix a registered service worker controls. By default the scope equals the path the worker file is served from; the Service-Worker-Allowed header can broaden it.

• Shadow root

  An encapsulated DOM tree attached to a host element via attachShadow(). Styles inside the shadow root do not cascade out, and outside selectors do not match into it; CSS custom properties pierce the boundary.

• Specificity

  The CSS scoring of a selector that resolves cascade ties when origin, importance, and layer are equal. Counted as a four-part tuple: inline, ids, classes/attributes/pseudo-classes, type/pseudo-element.

• Tab order

  The sequence in which interactive elements receive keyboard focus when the user presses Tab. By default, follows DOM order; elements with positive tabindex jump ahead, which is rarely what authors intend.

• User-agent stylesheet

  The default CSS bundled with the browser that applies before any author or user style. Sets the baseline rendering of every HTML element; differs subtly across Chromium, WebKit, and Gecko.