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183 lines
9.3 KiB
Markdown
# Accessibility in Angular
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The web is used by a wide variety of people, including those who have visual or motor impairments.
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A variety of assistive technologies are available that make it much easier for these groups to interact with web-based software applications.
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In addition, designing an application to be more accessible generally improves the user experience for all users.
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For an in-depth introduction to issues and techniques for designing accessible applications, see the [Accessibility](https://developers.google.com/web/fundamentals/accessibility/#what_is_accessibility) section of the Google's [Web Fundamentals](https://developers.google.com/web/fundamentals).
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This page discusses best practices for designing Angular applications that work well for all users, including those who rely on assistive technologies.
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<div class="alert is-helpful">
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For the sample application that this page describes, see the <live-example></live-example>.
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</div>
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## Accessibility attributes
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Building accessible web experience often involves setting [ARIA attributes](https://developers.google.com/web/fundamentals/accessibility/semantics-aria) to provide semantic meaning where it might otherwise be missing.
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Use [attribute binding](guide/attribute-binding) template syntax to control the values of accessibility-related attributes.
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When binding to ARIA attributes in Angular, you must use the `attr.` prefix, as the ARIA specification depends specifically on HTML attributes rather than properties of DOM elements.
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<code-example format="html" language="html">
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<!-- Use attr. when binding to an ARIA attribute -->
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<button [attr.aria-label]="myActionLabel">…</button>
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</code-example>
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<div class="alert is-helpful">
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**NOTE** <br />
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This syntax is only necessary for attribute *bindings*.
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Static ARIA attributes require no extra syntax.
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<code-example format="html" language="html">
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<!-- Static ARIA attributes require no extra syntax -->
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<button aria-label="Save document">…</button>
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</code-example>
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</div>
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<div class="alert is-helpful">
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By convention, HTML attributes use lowercase names \(`tabindex`\), while properties use camelCase names \(`tabIndex`\).
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See the [Binding syntax](guide/binding-syntax#html-attribute-vs-dom-property) guide for more background on the difference between attributes and properties.
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</div>
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## Angular UI components
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The [Angular Material](https://material.angular.io) library, which is maintained by the Angular team, is a suite of reusable UI components that aims to be fully accessible.
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The [Component Development Kit (CDK)](https://material.angular.io/cdk/categories) includes the `a11y` package that provides tools to support various areas of accessibility.
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For example:
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* `LiveAnnouncer` is used to announce messages for screen-reader users using an `aria-live` region.
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See the W3C documentation for more information on [aria-live regions](https://www.w3.org/WAI/PF/aria-1.1/states_and_properties#aria-live).
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* The `cdkTrapFocus` directive traps Tab-key focus within an element.
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Use it to create accessible experience for components such as modal dialogs, where focus must be constrained.
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For full details of these and other tools, see the [Angular CDK accessibility overview](https://material.angular.io/cdk/a11y/overview).
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### Augmenting native elements
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Native HTML elements capture a number of standard interaction patterns that are important to accessibility.
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When authoring Angular components, you should re-use these native elements directly when possible, rather than re-implementing well-supported behaviors.
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For example, instead of creating a custom element for a new variety of button, create a component that uses an attribute selector with a native `<button>` element.
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This most commonly applies to `<button>` and `<a>`, but can be used with many other types of element.
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You can see examples of this pattern in Angular Material:
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[`MatButton`](https://github.com/angular/components/blob/50d3f29b6dc717b512dbd0234ce76f4ab7e9762a/src/material/button/button.ts#L67-L69), [`MatTabNav`](https://github.com/angular/components/blob/50d3f29b6dc717b512dbd0234ce76f4ab7e9762a/src/material/tabs/tab-nav-bar/tab-nav-bar.ts#L139), and [`MatTable`](https://github.com/angular/components/blob/50d3f29b6dc717b512dbd0234ce76f4ab7e9762a/src/material/table/table.ts#L22).
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### Using containers for native elements
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Sometimes using the appropriate native element requires a container element.
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For example, the native `<input>` element cannot have children, so any custom text entry components need to wrap an `<input>` with additional elements.
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While you might just include the `<input>` in your custom component's template, this makes it impossible for users of the component to set arbitrary properties and attributes to the input element.
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Instead, create a container component that uses content projection to include the native control in the component's API.
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You can see [`MatFormField`](https://material.angular.io/components/form-field/overview) as an example of this pattern.
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## Case study: Building a custom progress bar
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The following example shows how to make a progress bar accessible by using host binding to control accessibility-related attributes.
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* The component defines an accessibility-enabled element with both the standard HTML attribute `role`, and ARIA attributes.
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The ARIA attribute `aria-valuenow` is bound to the user's input.
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<code-example header="src/app/progress-bar.component.ts" path="accessibility/src/app/progress-bar.component.ts" region="progressbar-component"></code-example>
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* In the template, the `aria-label` attribute ensures that the control is accessible to screen readers.
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<code-example header="src/app/app.component.html" path="accessibility/src/app/app.component.html" region="template"></code-example>
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## Routing
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### Focus management after navigation
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Tracking and controlling [focus](https://developers.google.com/web/fundamentals/accessibility/focus) in a UI is an important consideration in designing for accessibility.
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When using Angular routing, you should decide where page focus goes upon navigation.
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To avoid relying solely on visual cues, you need to make sure your routing code updates focus after page navigation.
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Use the `NavigationEnd` event from the `Router` service to know when to update focus.
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The following example shows how to find and focus the main content header in the DOM after navigation.
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<code-example format="typescript" language="typescript">
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router.events.pipe(filter(e => e instanceof NavigationEnd)).subscribe(() => {
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const mainHeader = document.querySelector('#main-content-header')
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if (mainHeader) {
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mainHeader.focus();
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}
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});
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</code-example>
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In a real application, the element that receives focus depends on your specific application structure and layout.
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The focused element should put users in a position to immediately move into the main content that has just been routed into view.
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You should avoid situations where focus returns to the `body` element after a route change.
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### Active links identification
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CSS classes applied to active `RouterLink` elements (usually via `RouterLinkActive`) provide a visual indication regarding as to which link is currently active. Such indication doesn't apply to blind or visually impaired users, in order to provide such information the `aria-current` attribute should be applied to the element as well (for more information see [MDN aria-current](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-current)).
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The `RouterLinkActive` directive provides the `ariaCurrentWhenActive` input which sets the `aria-current` to a specified value when the link becomes active.
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The following example shows how to apply the `active-page` class to active links as well as setting their `aria-current` attribute to `"page"` when they are active:
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```html
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<nav>
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<a routerLink="home"
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routerLinkActive="active-page"
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ariaCurrentWhenActive="page">
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Home
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</a>
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<a routerLink="about"
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routerLinkActive="active-page"
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ariaCurrentWhenActive="page">
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About
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</a>
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<a routerLink="shop"
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routerLinkActive="active-page"
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ariaCurrentWhenActive="page">
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Shop
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</a>
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</nav>
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```
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## Additional resources
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* [Accessibility - Google Web Fundamentals](https://developers.google.com/web/fundamentals/accessibility)
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* [ARIA specification and authoring practices](https://www.w3.org/TR/wai-aria)
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* [Material Design - Accessibility](https://material.io/design/usability/accessibility.html)
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* [Smashing Magazine](https://www.smashingmagazine.com/search/?q=accessibility)
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* [Inclusive Components](https://inclusive-components.design)
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* [Accessibility Resources and Code Examples](https://dequeuniversity.com/resources)
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* [W3C - Web Accessibility Initiative](https://www.w3.org/WAI/people-use-web)
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* [Rob Dodson A11ycasts](https://www.youtube.com/watch?v=HtTyRajRuyY)
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* [Angular ESLint](https://github.com/angular-eslint/angular-eslint#functionality) provides linting rules that can help you make sure your code meets accessibility standards.
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Books
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<!-- vale Angular.Google_Quotes = NO -->
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* "A Web for Everyone: Designing Accessible User Experiences," Sarah Horton and Whitney Quesenbery
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* "Inclusive Design Patterns," Heydon Pickering
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<!-- vale Angular.Google_Quotes = YES -->
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<!-- links -->
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<!-- external links -->
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<!-- end links -->
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@reviewed 2022-02-28
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