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 aname
attribute value ofviewport
; and - the attribute value has at least one of the
maximum-scale
oruser-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; oruser-scalable
isyes
,device-width
ordevice-height
; oruser-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; ormaximum-scale
isdevice-width
ordevice-height
; ormaximum-scale
is a negative number; ormaximum-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
- Understanding Success Criterion 1.4.4: Resize text
- Understanding Success Criterion 1.4.10: Reflow
- HTML Specification - The
meta
element - The initial-scale, minimum-scale, and maximum-scale properties
- The user-scalable property
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
- Any
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.
Test Cases
Passed
Passed Example 1
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
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
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
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
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
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 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
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
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
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
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
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
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
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
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
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 eitherImage Button
(the state) orimage
(the keyword mapping to it), both formulations having the same meaning; similarly, “an input element with atype
attribute value ofText
” 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 andfalse
otherwise. Thus<button disabled>
,<button disabled="disabled">
and<button disabled="">
all have adisabled
attribute value oftrue
. - 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.
For more details, see examples of visible.
Rule Versions
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.