The required role attribute.
Required and optional aria-* states (e.g. aria-expanded, aria-selected, aria-current).
The full keyboard-interaction model (Tab / Arrow keys / Enter / Escape / Home / End / Type-ahead).
DOM structure conventions.
Common pitfalls.

When to use

Authoring a custom widget (Combobox, Tabs, Tree, Carousel, etc.).
Reviewing a third-party UI library's component for ARIA correctness.
Adding ARIA to an existing widget that was hand-rolled without it.
Configuring assertions in axe-a11y (or its pa11y reference) to enforce per-pattern conventions.

The first rule of ARIA

The APG (and WAI more broadly) consistently emphasizes:

No ARIA is better than bad ARIA. Native HTML elements with built-in semantics are preferred over custom widgets whenever possible.

In practice:

Native element	Don't replace with
`<button>`	`<div role="button">` - same semantics, more code, easier to break.
`<input type="checkbox">`	Custom checkbox unless visual constraints demand it.
`<a href>`	`<div onclick="navigate(...)">`.
`<select>`	Custom listbox UNLESS multi-select with rich content per option.
`<input type="radio">`	Custom radio group.

When the visual design demands a custom widget, follow the matching APG pattern exactly.

Canonical patterns (APG)

The 31 patterns documented per [apg-patterns][apg]:

Form controls

Pattern	When to use	Native fallback
Button	Trigger an action.	`<button>` - almost always.
Checkbox	Two-state binary; or three-state (mixed).	`<input type="checkbox">`.
Combobox	Searchable / filterable select with autocomplete.	`<select>` for plain selection only.
Disclosure	Show/hide content; one-way reveal.	`<details>` / `<summary>`.
Listbox	Multi-select OR rich-content single-select.	`<select>` for plain.
Menu / Menubar	Application menus (File / Edit / View bar).	(No native; APG required.)
Menu Button	Button that opens a menu.	(No native; APG required.)
Radio Group	Mutually-exclusive option set.	`<input type="radio" name="x">`.
Slider	Single-value within a range.	`<input type="range">`.
Slider (Multi-Thumb)	Range selection.	(No native; APG required.)
Spinbutton	Numeric input with increment/decrement controls.	`<input type="number">`.
Switch	On/off toggle, semantically distinct from checkbox.	(No native; APG required.)

Containers / navigation

Pattern	When to use
Accordion	Vertically-stacked list of headers; each expands.
Breadcrumb	Hierarchical-location indicator.
Carousel	Cycle through groups of equivalent content.
Dialog (Modal)	Overlay requiring user interaction. (See `wcag-keyboard-navigation` references/focus-trap.md.)
Alert Dialog	Modal that interrupts to convey urgency.
Tabs	Switch between sibling content sections.
Toolbar	Group of controls (buttons, dropdowns).
Tree View	Hierarchical navigation.
Treegrid	Hierarchical data grid (table rows that expand).
Window Splitter	Resize between two panes.

Feedback / indicators

Pattern	When to use
Alert	Important programmatically-determined message.
Feed	Dynamic stream of articles.
Grid	2D widget for navigating tabular UI elements.
Meter	Scalar measurement within a known range.
Tooltip	Brief contextual hint on hover/focus.

Content

Pattern	When to use
Landmarks	Navigation regions (`<nav>`, `<main>`, etc.).
Link	Navigation to a different resource.
Table	Tabular data (use `<table>`).

Per-pattern essentials

The APG's per-pattern documentation is detailed; for each pattern this skill highlights the load-bearing essentials.

Combobox

<label for="combo">Choose a fruit</label>
<input
  id="combo"
  role="combobox"
  aria-expanded="false"
  aria-controls="combo-listbox"
  aria-autocomplete="list"
  aria-activedescendant=""
/>
<ul id="combo-listbox" role="listbox" hidden>
  <li id="opt-1" role="option" aria-selected="false">Apple</li>
  <li id="opt-2" role="option" aria-selected="false">Banana</li>
</ul>

Required ARIA	Why
`role="combobox"`	Identifies the input as a combobox.
`aria-expanded`	Tracks open/closed state.
`aria-controls`	Links to the listbox ID.
`aria-autocomplete`	`none` / `inline` / `list` / `both`.
`aria-activedescendant`	Tracks the focused option without moving DOM focus.

Tabs

<div role="tablist" aria-label="Settings sections">
  <button role="tab" id="tab-1" aria-selected="true" aria-controls="panel-1" tabindex="0">General</button>
  <button role="tab" id="tab-2" aria-selected="false" aria-controls="panel-2" tabindex="-1">Privacy</button>
</div>
<div role="tabpanel" id="panel-1" aria-labelledby="tab-1">...</div>
<div role="tabpanel" id="panel-2" aria-labelledby="tab-2" hidden>...</div>

Keyboard	Behavior
Tab	One tabstop for the whole tablist (only the active tab is `tabindex="0"`).
Left/Right arrow	Navigate between tabs.
Home / End	First / last tab.
Enter / Space	Activate (or auto on focus per implementation).

Disclosure (Show/Hide)

<button aria-expanded="false" aria-controls="disclosure-1">More details</button>
<div id="disclosure-1" hidden>... content ...</div>

The simplest pattern; canonical native equivalent is <details> / <summary> - use that unless the visual design demands the button form.

Tooltip (per SC 1.4.13)

<button aria-describedby="tt-1">Save</button>
<div id="tt-1" role="tooltip" hidden>Save the current document</div>

Behavior
Show on hover OR focus.
Dismissable via Esc (per SC 1.4.13).
Hoverable - user can move pointer to tooltip text.
Persistent until pointer / focus leaves OR Esc dismisses.

(See wcag-color-contrast SC 1.4.13 for the three conditions.)

ARIA states reference

The most-used aria-* states across patterns:

State	Use
`aria-expanded`	Disclosure / Accordion / Combobox / Menu Button.
`aria-selected`	Listbox / Tab.
`aria-checked`	Checkbox / Radio.
`aria-pressed`	Toggle button.
`aria-current`	Current item in a set (page in pagination, item in breadcrumb).
`aria-controls`	Element that this trigger controls.
`aria-labelledby`	Reference to the labelling element.
`aria-describedby`	Reference to a description element (tooltip, hint).
`aria-live`	Live region; `polite` / `assertive` / `off`.
`aria-invalid`	Form field validation state.
`aria-required`	Form field required (with native `required` for forms).
`aria-disabled`	Functionally disabled but in tab order (vs. `disabled` attribute which removes from tab order).
`aria-modal`	Dialog modal flag; see `wcag-keyboard-navigation` references/focus-trap.md.

Common ARIA failures

Failure	Why	Fix
`<div role="button">` without keyboard handlers	Custom button needs `tabindex="0"` AND Enter/Space handlers.	Use `<button>` OR add both.
`aria-label` on a `<div>` that has visible text label	Redundant; `aria-label` overrides the visible text for screen readers (becomes confusing).	Use `aria-labelledby` referencing the visible text element.
`aria-hidden="true"` on a focusable element	Element is hidden from screen readers but still in tab order - broken state.	If you want to hide, also remove from tab order.
Forgetting `role="tab"` / `role="tabpanel"` pairs	Screen reader doesn't announce the relationship.	Pair every tab with a panel via `aria-controls` / `aria-labelledby`.
Hand-rolled combobox without `aria-activedescendant`	Arrow keys don't announce options.	Implement `aria-activedescendant` or move actual DOM focus.
`role="presentation"` on a meaningful container	Removes semantics; sometimes used to "fix" a layout issue but breaks structure.	Don't override structure to fix layout; fix layout.

Anti-patterns

Anti-pattern	Why it fails	Fix
Inventing a custom pattern not in the APG	No screen-reader convention; users can't form expectations.	Match an APG pattern exactly OR fall back to native HTML.
Mixing native and ARIA semantics (`<button role="link">`)	Confusing - element behaves like a button but says it's a link.	One or the other; native is preferred.
`tabindex="3"` to control sequence	Positive tabindex creates fragile order.	DOM order + `tabindex="0"` for custom focusables.
Adding `aria-label` to every element "for accessibility"	If the element has a visible label, `aria-label` overrides it. Many elements don't need a label at all.	Only add `aria-label` when there's no visible text label AND the element is interactive.

References

[apg-patterns][apg] - W3C ARIA Authoring Practices Guide (canonical patterns).
WAI-ARIA 1.2 Spec - https://www.w3.org/TR/wai-aria-1.2/
wcag-keyboard-navigation - Keyboard-interaction conformance underlying every APG pattern; its references/focus-trap.md is the Dialog pattern's focus-management deep dive.
axe-a11y - scanners that detect ARIA misuse programmatically.

aria-authoring-patterns

aria-authoring-patterns

Overview