Device motion based changes to the content can be disabled

  • Rule Type:atomic
  • Rule Id: c249d5
  • Last modified: Jul 08, 2021
  • Accessibility Requirements Mapping:
    • 2.5.4 Motion Actuation (Level A)
      • Learn More about 2.5.4 Motion Actuation
      • Required for conformance to WCAG 2.1 on level A and higher.
      • Outcome mapping:
        • Any failed outcomes: success criterion is not satisfied.
        • All passed outcomes: success criterion needs further testing.
        • An inapplicable outcome: success criterion needs further testing.

Description

This rule checks that it is possible to disable any changes to the content of the web page resulting from device motion based events.

Applicability

This rule applies to an HTML document with an associated Window object that has an event listener list with one or more event listeners for device orientation events or device motion events.

Expectation

For each registered device orientation event or device motion event in the test target at least one of the following is true:

Assumptions

  • The motion to operate the device is not used through an accessibility supported interface, which is listed as a valid exception to Success Criterion 2.5.4: Motion Actuation.
  • The motion is not essential for the functionality it triggers, which is listed as a valid exception to Success Criterion 2.5.4: Motion Actuation.
  • The event listeners listening to device motion events trigger a functionality in the web page. If they do not trigger any such functionality failing this rule might not be a failure of the success criterion.
  • If there are ways to disable the device motion based functionality that do not require the user to interact with the web page (e.g. a setting at the operating system level), failing this rule might not be a failure of the success criterion.
  • This rule assumes that there are no changes in the content of the web page caused by another event. If this is not the case, changes may be attributed to the wrong event and the rule may fail while Success Criterion 2.5.4: Motion Actuation is still satisfied.
  • This rule assumes that the changes happen within a 1 minute time span after the event firing and therefore the comparison between the page before and after the event firing can be made at any time after that time span elapses. If there are changes after this time span, they may not be detected as changes in content and the rule may pass but Success Criterion 2.5.4: Motion Actuation is not satisfied. The arbitrary 1 minute time span, selected so that testing this rule would not be impractical, is not included in WCAG.
  • After being disabled, the event remains disabled until being re-enabled again. If the event is re-enabled through other non-user controlled means (e.g. a timeout) then this rule may pass while Success Criterion 2.5.4: Motion Actuation is not satisfied.

Accessibility Support

There are no major accessibility support issues known for this rule.

Background

The instruments used to pass this rule (if any), must meet all level A Success Criteria in order to fully satisfy Success Criterion 2.5.4: Motion Actuation. These extra requirements are left out of this rule, and should be tested separately.

Test Cases

Passed

Passed Example 1

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This HTML document has device orientation events that cause no changes to the content of the web page.

<html>
	<head>
		<title>Passed Example 1</title>
		<script>
			function activateEvent() {
				let counter = 0
				window.addEventListener('deviceorientation', () => {
					counter++
				})
			}
		</script>
	</head>

	<body onload="activateEvent();">
		<p>ACT-R</p>
		<p>Note: This example may not work across all browsers.</p>
	</body>
</html>

Passed Example 2

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This HTML document that can be operated through the device's orientation to increase and decrease the value of a slider has a control to disable that functionality.

<html>
	<head>
		<title>Passed Example 2</title>
		<script src="/test-assets/7677a9/slider.js"></script>
		<script>
			function activateSlider() {
				window.addEventListener('deviceorientation', handleOrientationCanBeDisabled)
			}
		</script>
	</head>

	<body onload="activateSlider();">
		<h1>Slider Motion Sensor Example</h1>

		<p>
			Open this slider on a device with a motion sensor, such as a smartphone or tablet. Tilt the device to the right
			and left to adjust the slider value. The check box disables the motion sensing adjustment.
		</p>
		<p>Note: This example may not work across all browsers.</p>

		<div>
			<input type="range" min="1" max="100" value="50" id="motionSlider" disabled />
			<p aria-live="polite">Slider Value: <span id="output">50</span></p>
		</div>
		<div>
			<input type="checkbox" id="disableMotion" />
			<label for="disableMotion">Disable Motion Actuation</label>
		</div>
	</body>
</html>

Passed Example 3

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This HTML document that can be operated by rotating the device to increase and decrease the value of a slider has a control to disable that functionality.

<html>
	<head>
		<title>Passed Example 3</title>
		<script src="/test-assets/7677a9/slider.js"></script>
		<script>
			function activateSlider() {
				window.addEventListener('devicemotion', handleMotionCanBeDisabled)
			}
		</script>
	</head>

	<body onload="activateSlider();">
		<h1>Slider Motion Sensor Example</h1>

		<p>
			Open this slider on a device with a motion sensor, such as a smart phone or tablet. Rotate the device to adjust
			the slider value. The check box disables the motion sensing adjustment.
		</p>
		<p>Note: This example may not work across all browsers.</p>

		<div>
			<input type="range" min="1" max="100" value="50" id="motionSlider" disabled />
			<p aria-live="polite">Slider Value: <span id="output">50</span></p>
		</div>
		<div>
			<input type="checkbox" id="disableMotion" />
			<label for="disableMotion">Disable Motion Actuation</label>
		</div>
	</body>
</html>

Failed

Failed Example 1

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This HTML document that can be operated through the device's orientation to increase and decrease the value of a slider but has no way to disable this functionality.

<html>
	<head>
		<title>Failed Example 1</title>
		<script src="/test-assets/7677a9/slider.js"></script>
		<script>
			function activateSlider() {
				window.addEventListener('deviceorientation', handleOrientation)
			}
		</script>
	</head>

	<body onload="activateSlider();">
		<pre class="output"></pre>

		<h1>Slider Motion Sensor Example</h1>

		<p>
			Open this slider on a device with a motion sensor, such as a smartphone or tablet. Tilt the device to the right
			and left to adjust the slider value.
		</p>
		<p>Note: This example may not work across all browsers.</p>

		<div>
			<input type="range" min="1" max="100" value="50" id="motionSlider" disabled />
			<button id="increaseSlider" type="button">Increase Value</button>
			<p aria-live="polite">Slider Value: <span id="output">50</span></p>
		</div>
	</body>
</html>

Inapplicable

Inapplicable Example 1

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This HTML document is not operable by device motion.

<p>ACT-Rules</p>

Glossary

Changes in content

A event originated change in the content of a web page occurs when, by comparing the web page before and 1 minute after the event firing, at least one of the following occurs:

  • visible changes: the rendered pixels change in any part of the document that is currently within the viewport or that can be brought into the viewport via scrolling; or
  • accessibility tree changes: any state, property or event of a node representing an accessible object of the accessibility tree changes, or any node is inserted in, or removed from the accessibility tree; or
  • audible changes: the audio rendered by the web page changes.
  • If the web page is rendering time-based media, rendered pixels and audio will be changing as part of the playback. The comparison in this instance should compare the pixels and audio that are rendered if the event is not fired, with the ones that are rendered if the event is fired.

Assumptions:

  • This definition assumes that there are no changes in the content of the web page caused by another event. If this is not the case, changes may be attributed to the wrong event.
  • This definition assumes that the changes happen within a 1 minute time span after the event firing and therefore the comparison between the page before and after the event firing can be made at any time after that time span elapses. If there are changes after this time span, this definition may not detect them. The arbitrary 1 minute time span, selected so that testing this rule would not be impractical, is not included in WCAG.

Clearly labeled location

Secondary information and alternative controls of functionality are often not displayed together with primary information or functionality. For example, an option to change a web page to dark mode may be placed on an options page instead of being available on every page and page state of a website. Another example is a maps application, where, instead of using GPS, an option is available in a dropdown menu to set the current location of the device. Such content should be placed in a clearly labeled location.

The location of a target is said to be clearly labeled when the target can be found by activating "identifiable" instruments which either lead the user to find the target, or to another page or page state from which this action can be repeated until the target is found.

Whether or not the content is "clearly labeled" depends on the starting point of the search. If page A has a link which clearly "identifies" some piece of content, then the location of the content is clearly labeled. Page B, which can be in the same website, may not have such a link or may have a link with a link text that does not "identify" target content or which can be interpreted to "identify" more than one target, and so the location of the content starting from page B is not clearly labeled.

For the purpose of this definition, an instrument is identifiable if any text or other content with a text alternative, allows any user to identify an element with a semantic role that inherits from widget.

A web page changes state when the document's body changes without a change in the document's URL.

Instrument to achieve an objective

An HTML element that when activated allows an end-user to achieve an objective.

Note: Any rule that uses this definition must provide an unambiguous description of the objective the instrument is used to achieve.

Background About Instrument for Instrument to achieve an objective

This definition is a more restrictive version of WCAG's definition of mechanism, notably restricting it to the current document. WCAG has a note that "The mechanism needs to meet all success criteria for the conformance level claimed." This includes all the level A criteria such as Success Criterion 2.1.1 Keyboard (the mechanism must be keyboard accessible) or Success Criterion 4.1.2 Name, Role, Value (the mechanism must be exposed to assistive technologies and have an accessible name). This definition, and the rules using it, leaves these extra requirements out. This avoids reporting the same component twice for the same reason (e.g., missing an accessible name) under two different rules and Success Criteria, and helps pinpoint the error related to each Success Criterion. Instruments should nonetheless be fully accessible at the correct conformance level (depending on the rule using them).

Outcome

An outcome is a conclusion that comes from evaluating an ACT Rule on a test subject or one of its constituent test target. An outcome can be one of the three following types:

  • Inapplicable: No part of the test subject matches the applicability
  • Passed: A test target meets all expectations
  • Failed: A test target does not meet all expectations

Note: A rule has one passed or failed outcome for every test target. When there are no test targets the rule has one inapplicable outcome. This means that each test subject will have one or more outcomes.

Note: Implementations using the EARL10-Schema can express the outcome with the outcome property. In addition to passed, failed and inapplicable, EARL 1.0 also defined an incomplete outcome. While this cannot be the outcome of an ACT Rule when applied in its entirety, it often happens that rules are only partially evaluated. For example, when applicability was automated, but the expectations have to be evaluated manually. Such "interim" results can be expressed with the incomplete outcome.

Web page (HTML)

An HTML web page is the set of all fully active documents which share the same top-level browsing context.

Note: Nesting of browsing context mostly happens with iframe and object. Thus a web page will most of the time be a "top-level" document and all its iframe and object (recursively).

Note: Web pages as defined by WCAG are not restricted to the HTML technology but can also include, e.g., PDF or DOCX documents.

Note: Although web pages as defined here are sets of documents (and do not contain other kind of nodes), one can abusively write that any node is "in a web page" if it is a shadow-including descendant of a document that is part of that web page.


Useful Links


Implementations

This section is not part of the official rule. It is populated dynamically and not accounted for in the change history or the last modified date. This section will not be included in the rule when it is published on the W3C website.

No Implementations

Implementation reports are not provided for this rule.

Acknowledgments

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