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Proposed Test Rule: Zoomed text node is not clipped with CSS overflow

Description

This rule checks that text nodes are not unintentionally clipped by overflow, when a page is zoomed to 200% on 1280 by 1024 viewport;

Applicability

This rule applies to any text node for which all of the following is true when in a viewport size of 640 by 512:

Note: A viewport size of 640 by 512 is equivalent to a viewport size of 1280 by 1024 zoomed 200%.

Expectation

Each test target is not clipped by overflow of an ancestor in the flat tree when in a viewport size of 640 by 512, except if the clipping ancestor has at least one of the following:

Assumptions

If any of the following assumptions is true, failing this rule may not result in a failure of success criterion 1.4.4 Resize text:

Accessibility Support

Some user agents treat the value of the aria-hidden attribute as case-sensitive.

Background

When the computed value of the line-height property is normal, the used value depends on font specific metrics. CSS specifications recommend that the used value is between 1.0 and 1.2 and major browsers are effectively using values close to 1.2.

Bibliography

Accessibility Requirements Mapping

Input Aspects

The following aspects are required in using this rule.

Test Cases

Passed

Passed Example 1

This text node is fully visible at a viewport size of 640 by 512.

<div style="white-space: nowrap; overflow: hidden; font-size: 16px;">
	Once upon a midnight dreary, while I pondered, weak and weary,<br />
	Over many a quaint and curious volume of forgotten lore.<br />
	While I nodded, nearly napping, suddenly there came a tapping,<br />
	As of some one gently rapping, rapping at my chamber door.<br />
	“’Tis some visitor,” I muttered, “tapping at my chamber door.<br />
	Only this and nothing more.”
</div>

Passed Example 2

This text node is clipped using text-overflow: ellipsis at a viewport size of 640 by 512. A link to a full version of the poem is also provided.

<div style="white-space: nowrap; overflow: hidden; text-overflow: ellipsis; font-size: 16px;">
	Once upon a midnight dreary, while I pondered, weak and weary, Over many a quaint and curious volume of forgotten
	lore. While I nodded, nearly napping, suddenly there came a tapping.
</div>
<a href="/test-assets/59br37/poem.html">Full text of the poem</a>

Passed Example 3

This text node is restricted to a single line, by setting a line-height that is the same as the height. A link to a full version of the poem is also provided.

<style>
	.wordClip {
		overflow: hidden;
		word-wrap: break-word;
		height: 16px;
		line-height: 16px;
		font-size: 16px;
	}
</style>
<div class="wordClip">
	Once upon a midnight dreary, while I pondered, weak and weary, Over many a quaint and curious volume of forgotten
	lore. While I nodded, nearly napping, suddenly there came a tapping.
</div>
<a href="/test-assets/59br37/poem.html">Full text of the poem</a>

Passed Example 4

This text node is not clipped with overflow: hidden because it has a parent with overflow: auto at a viewport size of 640 by 512.

<div style="overflow: hidden; height: 2em; font-size: 16px;">
	<div style="overflow: auto; height: 2em;">
		Once upon a midnight dreary, while I pondered, weak and weary, Over many a quaint and curious volume of forgotten
		lore. While I nodded, nearly napping, suddenly there came a tapping.
	</div>
</div>

Failed

Failed Example 1

This text node is clipped because it has a fixed height that does not leave enough space for the content to wrap.

<div style="overflow: hidden; height: 1.5em; font-size: 16px;">
	Once upon a midnight dreary, while I pondered, weak and weary, Over many a quaint and curious volume of forgotten
	lore. While I nodded, nearly napping, suddenly there came a tapping.
</div>

Failed Example 2

This text node is clipped because its height is relative to the viewport height, leaving insufficient space for the page to be zoomed to 200% at a viewport size of 1280 by 1024.

<div style="overflow: hidden; height: 16vh; font-size: 16px;">
	Once upon a midnight dreary, while I pondered, weak and weary,<br />
	Over many a quaint and curious volume of forgotten lore.<br />
	While I nodded, nearly napping, suddenly there came a tapping,<br />
	As of some one gently rapping, rapping at my chamber door.<br />
	“’Tis some visitor,” I muttered, “tapping at my chamber door.<br />
	Only this and nothing more.”
</div>

Failed Example 3

This text node is clipped by style that is applied at a viewport size width of 640.

<style>
	@media screen and (max-width: 640px) {
		.myContainer {
			height: 1.5em;
			width: 50%;
			overflow: hidden;
			font-size: 16px;
		}
	}
</style>
<div class="myContainer">
	Once upon a midnight dreary, while I pondered, weak and weary, Over many a quaint and curious volume of forgotten
	lore. While I nodded, nearly napping, suddenly there came a tapping, As of some one gently rapping, rapping at my
	chamber door. “’Tis some visitor,” I muttered, “tapping at my chamber door. Only this and nothing more.”
</div>

Inapplicable

Inapplicable Example 1

This text node is not visible at a viewport size of 640 by 512.

<p style="display:none;">Last updated 2020/03/27 10:52pm</p>

Inapplicable Example 2

This text node has an SVG element as a parent.

<svg>
	<text x="0" y="15">I love SVG!</text>
</svg>

Inapplicable Example 3

This text node has no ancestor with overflow: hidden or clip.

<div style="overflow: auto; height: 1.5em; font-size: 16px;">
	Once upon a midnight dreary, while I pondered, weak and weary, Over many a quaint and curious volume of forgotten
	lore. While I nodded, nearly napping, suddenly there came a tapping.
</div>

Inapplicable Example 4

This text node has an ancestor with aria-hidden set to true.

<img src="/test-assets/shared/w3c-logo.png" alt="W3C logo" /> <span aria-hidden="true">(W3C Logo)</span>

Inapplicable Example 5

This text node with the text “Web Content Accessibility Guidelines 2.1” is fully hidden in a viewport size of 640 by 512.

<style>
	@media screen and (max-width: 640px) {
		.mobile-hidden {
			position: absolute;
			width: 1px;
			height: 1px;
			overflow: hidden;
			font-size: 16px;
		}
	}
</style>
<a href="/"> Next<span class="mobile-hidden">: Web Content Accessibility Guidelines 2.1</span> </a>

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.

Clipped by Overflow

A node with an ancestor in the flat tree with a computed overflow of none or clipped, where changing the overflow of all such ancestors to visible would cause more of the node to become visible.

Namespaced Element

An element with a specific namespaceURI value from HTML namespaces. For example an “SVG element” is any element with the “SVG namespace”, which is http://www.w3.org/2000/svg.

Namespaced elements are not limited to elements described in a specification. They also include custom elements. Elements such as a and title have a different namespace depending on where they are used. For example a title in an HTML page usually has the HTML namespace. When used in an svg element, a title element has the SVG namespace instead.

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.

viewport size

The viewport size is the width and height at which a page is rendered. The viewport size is equal to the innerWidth and innerHeight of the window of the top-level browsing context.

Note: The viewport size is not to be confused with the “resolution” of the operating system. Often a browser will be a single window in the operating system, with a width and height different from the resolution of the operating system. Often browsers also include additional user interface components, such as a URL bar, tab bar, and a bookmarks bar. None of these are included in the viewport size. In full screen mode the viewport size might be the same as the resolution of the operating system.

Note: The viewport size includes, if rendered, all scrollbars.

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.

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 Consistency Complete Report
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Changelog

This is the first version of this ACT rule.

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This is an unpublished draft preview that might include content that is not yet approved. The published website is at w3.org/WAI/.