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Proposed Meta viewport allows for zoom

Description

This rule checks that the meta element retains the user agent ability to zoom.

Applicability

This rule applies to each content attribute on a meta element with a name attribute value of viewport for which at least one of the following is true:

Expectation 1

For each test target, the attribute value does not have a user-scalable property with a value of no.

Expectation 2

For each test target, the attribute value does not have a maximum-scale property with a value less than 2.

Assumptions

Pages for which any of the following is true may satisfy success criteria Success Criteria 1.4.4 Resize text and 1.4.10 Reflow, even if the rule results in a failed outcome.

Accessibility Support

Desktop browsers ignore the viewport meta element, and most modern mobile browsers either ignore it by default or have an accessibility option which will allow zooming. This rule is not relevant for desktop browsers, nor for most modern mobile browsers. Only users with older mobile browsers can experience issues tested by this rule.

Background

This rule is designed specifically for 1.4.4 Resize text, which requires that text can be resized up to 200%. Because text that can not be resized up to 200% can not fit in an area of 320 by 256 CSS pixels, this rule maps to 1.4.10 Reflow as well. All passed examples in this rule satisfy both success criteria.

Bibliography

Accessibility Requirements Mapping

Input Aspects

The following aspects are required in using this rule.

Test Cases

Passed

Passed Example 1

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This viewport meta element does not prevent user scaling because it has user-scalable set to yes.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="user-scalable=yes" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Passed Example 2

Open in a new tab

This viewport meta element allows users to scale content up to 200% because it has maximum-scale set to 2.0.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="maximum-scale=2.0" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Passed Example 3

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This viewport meta element does not prevent user scaling because it has maximum-scale set to -1 which results in this value being dropped.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="maximum-scale=-1" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Failed

Failed Example 1

Open in a new tab

This viewport meta element prevents user scaling because it has user-scalable set to no.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="user-scalable=no" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Failed Example 2

Open in a new tab

This viewport meta element prevents users to scale content up to 200% because it has maximum-scale set to 1.5.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="user-scalable=yes, initial-scale=0.8, maximum-scale=1.5" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Failed Example 3

Open in a new tab

This viewport meta element prevents users to scale content up to 200% because it has maximum-scale set to 1.0.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="maximum-scale=1.0" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Failed Example 4

Open in a new tab

This viewport meta element prevents users to scale content up to 200% because it has maximum-scale set to yes which translates to 1.0.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="maximum-scale=yes" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Inapplicable

Inapplicable Example 1

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There is no viewport meta element.

<html>
	<head>
		<title>Lorem ipsum</title>
		<meta charset="UTF-8" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Inapplicable Example 2

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This viewport meta element does not have a content attribute.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Inapplicable Example 3

Open in a new tab

This viewport meta element does not specify the maximum-scale nor user-scalable values.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="width=device-width" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Inapplicable Example 4

Open in a new tab

This viewport meta element does not prevent user scaling because it does not specify the maximum-scale nor user-scalable values.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Glossary

Attribute value

The attribute value of a content attribute set on an HTML element is the value that the attribute gets after being parsed and computed according to specifications. It may differ from the value that is actually written in the HTML code due to trimming whitespace or non-digits characters, default values, or case-insensitivity.

Some notable case of attribute value, among others:

This list is not exhaustive, and only serves as an illustration for some of the most common cases.

The attribute value of an IDL attribute is the value returned on getting it. Note that when an IDL attribute reflects a content attribute, they have the same attribute value.

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:

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.

Visible

Content perceivable through sight.

Content is considered visible if making it fully transparent would result in a difference in the pixels rendered for any part of the document that is currently within the viewport or can be brought into the viewport via scrolling.

Content is defined in WCAG.

For more details, see examples of visible.

Rule Versions

  1. Latest version, 25 October 2022

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.

Implementation Type Consistency Report
Alfa 0.56.0 Automated tool Consistent Alfa Report
Axe DevTools Pro 4.37.1 Semi-automated tool Consistent Axe DevTools Pro Report
Axe-core 4.6.0 Automated tool Consistent Axe-core Report
Equal Access Accessibility Checker 3.1.42-rc.0 Automated tool Partial Equal Access Accessibility Checker Report
QualWeb 3.0.0 Automated tool Consistent QualWeb Report
SortSite 6.45 Automated tool Consistent SortSite Report
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