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 for which all of the following are true:

the attribute is defined on a meta element with a name attribute value of viewport; and
the attribute value has at least one of the maximum-scale or user-scalable keys.

For the purpose of this rule, the attribute value of the content attribute is a list of key/value pairs.

Expectation 1

For each test target’s attribute value, at least one of the following is true:

user-scalable is not defined; or
user-scalable is yes, device-width or device-height; or
user-scalable is a number which is not between -1 and 1 (excluded).

Note: This is equivalent to applying the translations into a @viewport descriptors and not obtaining a value of fixed for the user-zoom descriptor from the translation for user-scalable.

Expectation 2

For each test target’s attribute value, at least one of the following is true:

maximum-scale is not defined; or
maximum-scale is device-width or device-height; or
maximum-scale is a negative number; or
maximum-scale is a number which is 2 or more.

Note: This is equivalent to applying the translations into a @viewport descriptors and not obtaining a value smaller than 2 for the max-zoom descriptor from the translation for maximum-scale.

Assumptions

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

The page has no visible content; or
There is another mechanism available to resize the text content; or
The content does not need to reflow in order to fit in an area of 320 by 256 CSS pixels.

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.

The exact way the content attribute should be parsed (notably, for error handling) is not fully specified. CSS specification includes a non-normative parsing algorithm. Different user agents may behave differently in some cases.

Background

Bibliography

Accessibility Requirements Mapping

1.4.4 Resize text (Level AA)
- Learn more about 1.4.4 Resize text
- 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

Secondary Requirements

This rule is related to the following accessibility requirements, but was not designed to test this requirements directly. These secondary requirements can either be stricter than the rule requires, or may be satisfied in ways not tested by the rule:

1.4.10 Reflow (Level AA): This success criterion is related to this rule. This is because a page that cannot be zoomed up to 200% often does not reflow sufficiently either. Most failed examples satisfy this success criterion.

Input Aspects

The following aspects are required in using this rule.

DOM Tree

Test Cases

Passed

Passed Example 1

Open in a new tab

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 does not prevent user scaling because it has user-scalable set to 5.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="user-scalable=5" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Passed Example 3

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 4

Open in a new tab

This viewport meta element does not prevent user scaling because it has maximum-scale set to -1, a negative value.

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

Passed Example 5

Open in a new tab

This viewport meta element does not prevent user scaling because it has maximum-scale set to device-width.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="maximum-scale=device-width" />
	</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 user scaling because it has user-scalable set to 0.5.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="user-scalable=0.5" />
	</head>
	<body>
		<p>
			Lorem ipsum
		</p>
	</body>
</html>

Failed Example 3

Open in a new tab

This viewport meta element prevents user scaling because it has user-scalable set to invalid.

<html>
	<head>
		<title>Simple page showing random text</title>
		<meta name="viewport" content="user-scalable=invalid" />
	</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 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 5

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.

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

Failed Example 6

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.

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

Failed Example 7

Open in a new tab

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

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

Inapplicable

Inapplicable Example 1

Open in a new tab

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

Open in a new tab

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:

For enumerated attributes, the attribute value is either the state of the attribute, or the keyword that maps to it; even for the default states. Thus <input type="image" /> has an attribute value of either Image Button (the state) or image (the keyword mapping to it), both formulations having the same meaning; similarly, “an input element with a type attribute value of Text” can be either <input type="text" />, <input /> (missing value default), or <input type="invalid" /> (invalid value default).
For boolean attributes, the attribute value is true when the attribute is present and false otherwise. Thus <button disabled>, <button disabled="disabled"> and <button disabled=""> all have a disabled attribute value of true.
For attributes whose value is used in a case-insensitive context, the attribute value is the lowercase version of the value written in the HTML code.
For attributes that accept numbers, the attribute value is the result of parsing the value written in the HTML code according to the rules for parsing this kind of number.
For attributes that accept sets of tokens, whether space separated or comma separated, the attribute value is the set of tokens obtained after parsing the set and, depending on the case, converting its items to lowercase (if the set is used in a case-insensitive context).
For aria-* attributes, the attribute value is computed as indicated in the WAI-ARIA specification and the HTML Accessibility API Mappings.

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:

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.

Rule Versions

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
Alfa (fully automated) 0.57.2	Automated tool	Consistent
Alfa (semi-automated) 0.57.2	Semi-automated tool	Consistent
Axe DevTools Pro 4.37.1	Semi-automated tool	Consistent
Axe-core 4.8.3	Automated tool	Consistent
Equal Access Accessibility Checker 3.1.42-rc.0	Automated tool	Partial
QualWeb 3.0.0	Automated tool	Consistent
SortSite 6.45	Automated tool	Consistent
Total Validator 17.4.0	Linter	Consistent
Total Validator (+Browser) 17.4.0	Automated tool	Consistent

Description

Applicability

Expectation 1

Expectation 2

Assumptions

Accessibility Support

Background

Bibliography

Accessibility Requirements Mapping

Secondary Requirements

Input Aspects

Test Cases

Passed

Passed Example 1

Passed Example 2

Passed Example 3

Passed Example 4

Passed Example 5

Failed

Failed Example 1

Failed Example 2

Failed Example 3

Failed Example 4

Failed Example 5

Failed Example 6

Failed Example 7

Inapplicable

Inapplicable Example 1

Inapplicable Example 2

Inapplicable Example 3

Inapplicable Example 4

Glossary

Attribute value

Outcome

Visible

Rule Versions

Implementations

Useful Links