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361 lines
14 KiB
361 lines
14 KiB
---
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title: 'Accessibility warnings'
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---
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Accessibility (shortened to a11y) isn't always easy to get right, but Svelte will help by warning you at compile time if you write inaccessible markup. However, keep in mind that many accessibility issues can only be identified at runtime using other automated tools and by manually testing your application.
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Some warnings may be incorrect in your concrete use case. You can disable such false positives by placing a `<!-- svelte-ignore a11y-<code> -->` comment above the line that causes the warning. Example:
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```svelte
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<!-- svelte-ignore a11y-autofocus -->
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<input autofocus />
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```
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Here is a list of accessibility checks Svelte will do for you.
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## `a11y-accesskey`
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Enforce no `accesskey` on element. Access keys are HTML attributes that allow web developers to assign keyboard shortcuts to elements. Inconsistencies between keyboard shortcuts and keyboard commands used by screen reader and keyboard-only users create accessibility complications. To avoid complications, access keys should not be used.
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<!-- prettier-ignore -->
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```svelte
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<!-- A11y: Avoid using accesskey -->
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<div accessKey="z" />
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```
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## `a11y-aria-activedescendant-has-tabindex`
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An element with `aria-activedescendant` must be tabbable, so it must either have an inherent `tabindex` or declare `tabindex` as an attribute.
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```svelte
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<!-- A11y: Elements with attribute aria-activedescendant should have tabindex value -->
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<div aria-activedescendant="some-id" />
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```
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## `a11y-aria-attributes`
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Certain reserved DOM elements do not support ARIA roles, states and properties. This is often because they are not visible, for example `meta`, `html`, `script`, `style`. This rule enforces that these DOM elements do not contain the `aria-*` props.
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```svelte
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<!-- A11y: <meta> should not have aria-* attributes -->
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<meta aria-hidden="false" />
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```
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## `a11y-autofocus`
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Enforce that `autofocus` is not used on elements. Autofocusing elements can cause usability issues for sighted and non-sighted users alike.
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```svelte
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<!-- A11y: Avoid using autofocus -->
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<input autofocus />
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```
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## `a11y-click-events-have-key-events`
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Enforce that visible, non-interactive elements with an `on:click` event are accompanied by a keyboard event handler.
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Users should first consider whether an interactive element might be more appropriate such as a `<button type="button">` element for actions or `<a>` element for navigations. These elements are more semantically meaningful and will have built-in key handling. E.g. `Space` and `Enter` will trigger a `<button>` and `Enter` will trigger an `<a>` element.
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If a non-interactive element is required then `on:click` should be accompanied by an `on:keyup` or `on:keydown` handler that enables the user to perform equivalent actions via the keyboard. In order for the user to be able to trigger a key press, the element will also need to be focusable by adding a [`tabindex`](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/tabindex). While an `on:keypress` handler will also silence this warning, it should be noted that the `keypress` event is deprecated.
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```svelte
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<!-- A11y: visible, non-interactive elements with an on:click event must be accompanied by a keyboard event handler. -->
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<div on:click={() => {}} />
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```
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Coding for the keyboard is important for users with physical disabilities who cannot use a mouse, AT compatibility, and screenreader users.
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## `a11y-distracting-elements`
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Enforces that no distracting elements are used. Elements that can be visually distracting can cause accessibility issues with visually impaired users. Such elements are most likely deprecated, and should be avoided.
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The following elements are visually distracting: `<marquee>` and `<blink>`.
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```svelte
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<!-- A11y: Avoid <marquee> elements -->
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<marquee />
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```
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## `a11y-hidden`
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Certain DOM elements are useful for screen reader navigation and should not be hidden.
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<!-- prettier-ignore -->
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```svelte
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<!-- A11y: <h2> element should not be hidden -->
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<h2 aria-hidden="true">invisible header</h2>
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```
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## `a11y-img-redundant-alt`
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Enforce img alt attribute does not contain the word image, picture, or photo. Screen readers already announce `img` elements as an image. There is no need to use words such as _image_, _photo_, and/or _picture_.
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```svelte
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<img src="foo" alt="Foo eating a sandwich." />
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<!-- aria-hidden, won't be announced by screen reader -->
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<img src="bar" aria-hidden="true" alt="Picture of me taking a photo of an image" />
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<!-- A11y: Screen readers already announce <img> elements as an image. -->
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<img src="foo" alt="Photo of foo being weird." />
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<!-- A11y: Screen readers already announce <img> elements as an image. -->
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<img src="bar" alt="Image of me at a bar!" />
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<!-- A11y: Screen readers already announce <img> elements as an image. -->
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<img src="foo" alt="Picture of baz fixing a bug." />
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```
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## `a11y-incorrect-aria-attribute-type`
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Enforce that only the correct type of value is used for aria attributes. For example, `aria-hidden`
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should only receive a boolean.
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```svelte
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<!-- A11y: The value of 'aria-hidden' must be exactly one of true or false -->
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<div aria-hidden="yes" />
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```
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## `a11y-invalid-attribute`
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Enforce that attributes important for accessibility have a valid value. For example, `href` should not be empty, `'#'`, or `javascript:`.
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```svelte
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<!-- A11y: '' is not a valid href attribute -->
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<a href="">invalid</a>
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```
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## `a11y-interactive-supports-focus`
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Enforce that elements with an interactive role and interactive handlers (mouse or key press) must be focusable or tabbable.
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```svelte
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<!-- A11y: Elements with the 'button' interactive role must have a tabindex value. -->
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<div role="button" on:keypress={() => {}} />
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```
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## `a11y-label-has-associated-control`
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Enforce that a label tag has a text label and an associated control.
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There are two supported ways to associate a label with a control:
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- Wrapping a control in a label tag.
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- Adding `for` to a label and assigning it the ID of an input on the page.
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```svelte
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<label for="id">B</label>
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<label>C <input type="text" /></label>
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<!-- A11y: A form label must be associated with a control. -->
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<label>A</label>
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```
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## `a11y-media-has-caption`
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Providing captions for media is essential for deaf users to follow along. Captions should be a transcription or translation of the dialogue, sound effects, relevant musical cues, and other relevant audio information. Not only is this important for accessibility, but can also be useful for all users in the case that the media is unavailable (similar to `alt` text on an image when an image is unable to load).
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The captions should contain all important and relevant information to understand the corresponding media. This may mean that the captions are not a 1:1 mapping of the dialogue in the media content. However, captions are not necessary for video components with the `muted` attribute.
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```svelte
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<video><track kind="captions" /></video>
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<audio muted />
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<!-- A11y: Media elements must have a <track kind=\"captions\"> -->
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<video />
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<!-- A11y: Media elements must have a <track kind=\"captions\"> -->
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<video><track /></video>
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```
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## `a11y-misplaced-role`
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Certain reserved DOM elements do not support ARIA roles, states and properties. This is often because they are not visible, for example `meta`, `html`, `script`, `style`. This rule enforces that these DOM elements do not contain the `role` props.
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```svelte
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<!-- A11y: <meta> should not have role attribute -->
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<meta role="tooltip" />
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```
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## `a11y-misplaced-scope`
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The scope attribute should only be used on `<th>` elements.
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<!-- prettier-ignore -->
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```svelte
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<!-- A11y: The scope attribute should only be used with <th> elements -->
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<div scope="row" />
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```
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## `a11y-missing-attribute`
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Enforce that attributes required for accessibility are present on an element. This includes the following checks:
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- `<a>` should have an href (unless it's a [fragment-defining tag](https://github.com/sveltejs/svelte/issues/4697))
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- `<area>` should have alt, aria-label, or aria-labelledby
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- `<html>` should have lang
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- `<iframe>` should have title
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- `<img>` should have alt
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- `<object>` should have title, aria-label, or aria-labelledby
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- `<input type="image">` should have alt, aria-label, or aria-labelledby
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```svelte
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<!-- A11y: <input type=\"image\"> element should have an alt, aria-label or aria-labelledby attribute -->
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<input type="image" />
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<!-- A11y: <html> element should have a lang attribute -->
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<html />
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<!-- A11y: <a> element should have an href attribute -->
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<a>text</a>
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```
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## `a11y-missing-content`
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Enforce that heading elements (`h1`, `h2`, etc.) and anchors have content and that the content is accessible to screen readers
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```svelte
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<!-- A11y: <a> element should have child content -->
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<a href="/foo" />
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<!-- A11y: <h1> element should have child content -->
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<h1 />
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```
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## `a11y-mouse-events-have-key-events`
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Enforce that `on:mouseover` and `on:mouseout` are accompanied by `on:focus` and `on:blur`, respectively. This helps to ensure that any functionality triggered by these mouse events is also accessible to keyboard users.
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```svelte
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<!-- A11y: on:mouseover must be accompanied by on:focus -->
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<div on:mouseover={handleMouseover} />
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<!-- A11y: on:mouseout must be accompanied by on:blur -->
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<div on:mouseout={handleMouseout} />
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```
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## `a11y-no-redundant-roles`
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Some HTML elements have default ARIA roles. Giving these elements an ARIA role that is already set by the browser [has no effect](https://www.w3.org/TR/using-aria/#aria-does-nothing) and is redundant.
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```svelte
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<!-- A11y: Redundant role 'button' -->
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<button role="button" />
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<!-- A11y: Redundant role 'img' -->
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<img role="img" src="foo.jpg" />
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```
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## `a11y-no-interactive-element-to-noninteractive-role`
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[WAI-ARIA](https://www.w3.org/TR/wai-aria-1.1/#usage_intro) roles should not be used to convert an interactive element to a non-interactive element. Non-interactive ARIA roles include `article`, `banner`, `complementary`, `img`, `listitem`, `main`, `region` and `tooltip`.
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```svelte
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<!-- A11y: <textarea> cannot have role 'listitem' -->
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<textarea role="listitem" />
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```
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### `a11y-no-noninteractive-element-interactions`
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A non-interactive element does not support event handlers (mouse and key handlers). Non-interactive elements include `<main>`, `<area>`, `<h1>` (,`<h2>`, etc), `<p>`, `<img>`, `<li>`, `<ul>` and `<ol>`. Non-interactive [WAI-ARIA roles](https://www.w3.org/TR/wai-aria-1.1/#usage_intro) include `article`, `banner`, `complementary`, `img`, `listitem`, `main`, `region` and `tooltip`.
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```sv
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<!-- `A11y: Non-interactive element <li> should not be assigned mouse or keyboard event listeners.` -->
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<li on:click={() => {}} />
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<!-- `A11y: Non-interactive element <div> should not be assigned mouse or keyboard event listeners.` -->
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<div role="listitem" on:click={() => {}} />
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```
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### `a11y-no-noninteractive-element-to-interactive-role`
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[WAI-ARIA](https://www.w3.org/TR/wai-aria-1.1/#usage_intro) roles should not be used to convert a non-interactive element to an interactive element. Interactive ARIA roles include `button`, `link`, `checkbox`, `menuitem`, `menuitemcheckbox`, `menuitemradio`, `option`, `radio`, `searchbox`, `switch` and `textbox`.
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```svelte
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<!-- A11y: Non-interactive element <h3> cannot have interactive role 'searchbox' -->
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<h3 role="searchbox">Button</h3>
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```
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## `a11y-no-noninteractive-tabindex`
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Tab key navigation should be limited to elements on the page that can be interacted with.
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<!-- prettier-ignore -->
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```svelte
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<!-- A11y: noninteractive element cannot have nonnegative tabIndex value -->
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<div tabindex="0" />
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```
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## a11y-no-static-element-interactions
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Elements like `<div>` with interactive handlers like `click` must have an ARIA role.
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<!-- prettier-ignore -->
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```svelte
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<!-- A11y: <div> with click handler must have an ARIA role -->
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<div on:click={() => ''} />
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```
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## `a11y-positive-tabindex`
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Avoid positive `tabindex` property values. This will move elements out of the expected tab order, creating a confusing experience for keyboard users.
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<!-- prettier-ignore -->
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```svelte
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<!-- A11y: avoid tabindex values above zero -->
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<div tabindex="1" />
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```
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## `a11y-role-has-required-aria-props`
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Elements with ARIA roles must have all required attributes for that role.
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```svelte
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<!-- A11y: A11y: Elements with the ARIA role "checkbox" must have the following attributes defined: "aria-checked" -->
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<span role="checkbox" aria-labelledby="foo" tabindex="0" />
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```
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## `a11y-role-supports-aria-props`
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Elements with explicit or implicit roles defined contain only `aria-*` properties supported by that role.
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```svelte
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<!-- A11y: The attribute 'aria-multiline' is not supported by the role 'link'. -->
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<div role="link" aria-multiline />
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<!-- A11y: The attribute 'aria-required' is not supported by the role 'listitem'. This role is implicit on the element <li>. -->
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<li aria-required />
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```
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## `a11y-structure`
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Enforce that certain DOM elements have the correct structure.
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```svelte
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<!-- A11y: <figcaption> must be an immediate child of <figure> -->
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<div>
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<figcaption>Image caption</figcaption>
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</div>
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```
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## `a11y-unknown-aria-attribute`
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Enforce that only known ARIA attributes are used. This is based on the [WAI-ARIA States and Properties spec](https://www.w3.org/WAI/PF/aria-1.1/states_and_properties).
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```svelte
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<!-- A11y: Unknown aria attribute 'aria-labeledby' (did you mean 'labelledby'?) -->
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<input type="image" aria-labeledby="foo" />
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```
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## `a11y-unknown-role`
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Elements with ARIA roles must use a valid, non-abstract ARIA role. A reference to role definitions can be found at [WAI-ARIA](https://www.w3.org/TR/wai-aria/#role_definitions) site.
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<!-- prettier-ignore -->
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```svelte
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<!-- A11y: Unknown role 'toooltip' (did you mean 'tooltip'?) -->
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<div role="toooltip" />
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```
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