meta viewport allows for zoom

Rule Type:

atomic

Rule ID:

b4f0c3

Last Modified:

Oct 1, 2020

Accessibility Requirements Mapping:

1.4.4 Resize text (Level AA)

Required for conformance to WCAG 2.0 and later on level AA 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

1.4.10 Reflow (Level AA)

Required for conformance to WCAG 2.1 on level AA 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

Input Aspects:

DOM Tree

Description

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

Applicability

The rule applies to each meta element with a name attribute whose value is a case-insensitive match for viewport and has a content attribute.

Expectation

For each test target, the content attribute, whose value is mapped to a list of property/value pairs in a user-agent specific manner, does not:

specify the property user-scalable with a value of no; nor
specify the property maximum-scale with a value of less than 2.

Assumptions

If any of the following is false, this rule can fail while Success Criteria 1.4.4 Resize text and 1.4.10 Reflow can still be satisfied:

The page has visible content.
There is no other mechanism available to resize the text content.
The content is not by default rendered in a way that fits in an area of 320 by 256 CSS pixels, and needs to reflow to do so.

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

Test Cases

Passed

Passed Example 1

This viewport meta element does not prevent user scaling because it does not specify the maximum-scale and 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>

Passed Example 2

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 3

This viewport meta element allows users to scale content up to 600% because it has maximum-scale set to 6.0.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="maximum-scale=6.0" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Passed Example 4

This viewport meta element does not prevent user scaling because it does not specify the maximum-scale and user-scalable values.

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

Passed Example 5

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

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

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

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

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

There is no viewport meta element.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta http-equiv="refresh" content="10; URL='https://github.com'" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Inapplicable Example 2

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>

Glossary

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.

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.

Acknowledgements

This rule was written in the ACT Rules community group, with the support of the EU-funded WAI-Tools Project.

Authors

Changelog

This is the first version of this ACT rule.