svelte/documentation/docs/05-misc/02-accessibility-warnings.md

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Accessibility warnings

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.

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:

<!-- svelte-ignore a11y-autofocus -->
<input autofocus />

Here is a list of accessibility checks Svelte will do for you.

a11y-accesskey

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.

<!-- A11y: Avoid using accesskey -->
<div accessKey="z" />

a11y-aria-activedescendant-has-tabindex

An element with aria-activedescendant must be tabbable, so it must either have an inherent tabindex or declare tabindex as an attribute.

<!-- A11y: Elements with attribute aria-activedescendant should have tabindex value -->
<div aria-activedescendant="some-id" />

a11y-aria-attributes

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.

<!-- A11y: <meta> should not have aria-* attributes -->
<meta aria-hidden="false" />

a11y-autofocus

Enforce that autofocus is not used on elements. Autofocusing elements can cause usability issues for sighted and non-sighted users alike.

<!-- A11y: Avoid using autofocus -->
<input autofocus />

a11y-click-events-have-key-events

Enforce that visible, non-interactive elements with an on:click event are accompanied by a keyboard event handler.

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.

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. While an on:keypress handler will also silence this warning, it should be noted that the keypress event is deprecated.

<!-- A11y: visible, non-interactive elements with an on:click event must be accompanied by a keyboard event handler. -->
<div on:click={() => {}} />

Coding for the keyboard is important for users with physical disabilities who cannot use a mouse, AT compatibility, and screenreader users.

a11y-distracting-elements

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.

The following elements are visually distracting: <marquee> and <blink>.

<!-- A11y: Avoid <marquee> elements -->
<marquee />

a11y-hidden

Certain DOM elements are useful for screen reader navigation and should not be hidden.

<!-- A11y: <h2> element should not be hidden -->
<h2 aria-hidden="true">invisible header</h2>

a11y-img-redundant-alt

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.

<img src="foo" alt="Foo eating a sandwich." />

<!-- aria-hidden, won't be announced by screen reader -->
<img src="bar" aria-hidden="true" alt="Picture of me taking a photo of an image" />

<!-- A11y: Screen readers already announce <img> elements as an image. -->
<img src="foo" alt="Photo of foo being weird." />

<!-- A11y: Screen readers already announce <img> elements as an image. -->
<img src="bar" alt="Image of me at a bar!" />

<!-- A11y: Screen readers already announce <img> elements as an image. -->
<img src="foo" alt="Picture of baz fixing a bug." />

a11y-incorrect-aria-attribute-type

Enforce that only the correct type of value is used for aria attributes. For example, aria-hidden should only receive a boolean.

<!-- A11y: The value of 'aria-hidden' must be exactly one of true or false -->
<div aria-hidden="yes" />

a11y-invalid-attribute

Enforce that attributes important for accessibility have a valid value. For example, href should not be empty, '#', or javascript:.

<!-- A11y: '' is not a valid href attribute -->
<a href="">invalid</a>

a11y-interactive-supports-focus

Enforce that elements with an interactive role and interactive handlers (mouse or key press) must be focusable or tabbable.

<!-- A11y: Elements with the 'button' interactive role must have a tabindex value. -->
<div role="button" on:keypress={() => {}} />

a11y-label-has-associated-control

Enforce that a label tag has a text label and an associated control.

There are two supported ways to associate a label with a control:

• Wrapping a control in a label tag.
• Adding for to a label and assigning it the ID of an input on the page.

<label for="id">B</label>

<label>C <input type="text" /></label>

<!-- A11y: A form label must be associated with a control. -->
<label>A</label>

a11y-media-has-caption

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).

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.

<video><track kind="captions" /></video>

<audio muted />

<!-- A11y: Media elements must have a <track kind=\"captions\"> -->
<video />

<!-- A11y: Media elements must have a <track kind=\"captions\"> -->
<video><track /></video>

a11y-misplaced-role

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.

<!-- A11y: <meta> should not have role attribute -->
<meta role="tooltip" />

a11y-misplaced-scope

The scope attribute should only be used on <th> elements.

<!-- A11y: The scope attribute should only be used with <th> elements -->
<div scope="row" />

a11y-missing-attribute

Enforce that attributes required for accessibility are present on an element. This includes the following checks:

• <a> should have an href (unless it's a fragment-defining tag)
• <area> should have alt, aria-label, or aria-labelledby
• <html> should have lang
• <iframe> should have title
• <img> should have alt
• <object> should have title, aria-label, or aria-labelledby
• <input type="image"> should have alt, aria-label, or aria-labelledby

<!-- A11y: <input type=\"image\"> element should have an alt, aria-label or aria-labelledby attribute -->
<input type="image" />

<!-- A11y: <html> element should have a lang attribute -->
<html />

<!-- A11y: <a> element should have an href attribute -->
<a>text</a>

a11y-missing-content

Enforce that heading elements (h1, h2, etc.) and anchors have content and that the content is accessible to screen readers

<!-- A11y: <a> element should have child content -->
<a href="/foo" />

<!-- A11y: <h1> element should have child content -->
<h1 />

a11y-mouse-events-have-key-events

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.

<!-- A11y: on:mouseover must be accompanied by on:focus -->
<div on:mouseover={handleMouseover} />

<!-- A11y: on:mouseout must be accompanied by on:blur -->
<div on:mouseout={handleMouseout} />

a11y-no-redundant-roles

Some HTML elements have default ARIA roles. Giving these elements an ARIA role that is already set by the browser has no effect and is redundant.

<!-- A11y: Redundant role 'button' -->
<button role="button" />

<!-- A11y: Redundant role 'img' -->
<img role="img" src="foo.jpg" />

a11y-no-interactive-element-to-noninteractive-role

WAI-ARIA 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.

<!-- A11y: <textarea> cannot have role 'listitem' -->
<textarea role="listitem" />

a11y-no-noninteractive-element-interactions

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 include article, banner, complementary, img, listitem, main, region and tooltip.

<!-- `A11y: Non-interactive element <li> should not be assigned mouse or keyboard event listeners.` -->
<li on:click={() => {}} />

<!-- `A11y: Non-interactive element <div> should not be assigned mouse or keyboard event listeners.` -->
<div role="listitem" on:click={() => {}} />

a11y-no-noninteractive-element-to-interactive-role

WAI-ARIA 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.

<!-- A11y: Non-interactive element <h3> cannot have interactive role 'searchbox' -->
<h3 role="searchbox">Button</h3>

a11y-no-noninteractive-tabindex

Tab key navigation should be limited to elements on the page that can be interacted with.

<!-- A11y: noninteractive element cannot have nonnegative tabIndex value -->
<div tabindex="0" />

a11y-no-static-element-interactions

Elements like <div> with interactive handlers like click must have an ARIA role.

<!-- A11y: <div> with click handler must have an ARIA role -->
<div on:click={() => ''} />

a11y-positive-tabindex

Avoid positive tabindex property values. This will move elements out of the expected tab order, creating a confusing experience for keyboard users.

<!-- A11y: avoid tabindex values above zero -->
<div tabindex="1" />

a11y-role-has-required-aria-props

Elements with ARIA roles must have all required attributes for that role.

<!-- A11y: A11y: Elements with the ARIA role "checkbox" must have the following attributes defined: "aria-checked" -->
<span role="checkbox" aria-labelledby="foo" tabindex="0" />

a11y-role-supports-aria-props

Elements with explicit or implicit roles defined contain only aria-* properties supported by that role.

<!-- A11y: The attribute 'aria-multiline' is not supported by the role 'link'. -->
<div role="link" aria-multiline />

<!-- A11y: The attribute 'aria-required' is not supported by the role 'listitem'. This role is implicit on the element <li>. -->
<li aria-required />

a11y-structure

Enforce that certain DOM elements have the correct structure.

<!-- A11y: <figcaption> must be an immediate child of <figure> -->
<div>
	<figcaption>Image caption</figcaption>
</div>

a11y-unknown-aria-attribute

Enforce that only known ARIA attributes are used. This is based on the WAI-ARIA States and Properties spec.

<!-- A11y: Unknown aria attribute 'aria-labeledby' (did you mean 'labelledby'?) -->
<input type="image" aria-labeledby="foo" />

a11y-unknown-role

Elements with ARIA roles must use a valid, non-abstract ARIA role. A reference to role definitions can be found at WAI-ARIA site.

<!-- A11y: Unknown role 'toooltip' (did you mean 'tooltip'?) -->
<div role="toooltip" />