From c2cbfdfd38fe4603e017783402a6dfd0221f9fb6 Mon Sep 17 00:00:00 2001 From: "Patrick H. Lauke" Date: Sun, 12 Feb 2023 17:23:15 +0000 Subject: [PATCH] Port all changes to 1.4.6 Contrast (Enhanced) understanding as well --- understanding/20/contrast-enhanced.html | 151 +++++++++++------------- 1 file changed, 67 insertions(+), 84 deletions(-) diff --git a/understanding/20/contrast-enhanced.html b/understanding/20/contrast-enhanced.html index 8a2ad8a374..a34dcc4494 100644 --- a/understanding/20/contrast-enhanced.html +++ b/understanding/20/contrast-enhanced.html @@ -8,11 +8,9 @@

Understanding Contrast (Enhanced)

-

Intent of Contrast (Enhanced)

-

The intent of this Success Criterion is to provide enough contrast between text and its background so that it can be read by people with moderately low vision (who do not use contrast-enhancing assistive technology). For people without color deficiencies, @@ -27,7 +25,6 @@

Intent of Contrast (Enhanced)

words are used to create a background and the words could be rearranged or substituted without changing meaning, then it would be decorative and would not need to meet this criterion. -

Text that is larger and has wider character strokes is easier to read at lower contrast. @@ -37,53 +34,52 @@

Intent of Contrast (Enhanced)

enough to require a lower contrast ratio. (See The American Printing House for the Blind Guidelines for Large Printing and The Library of Congress Guidelines for Large Print under - Resources). "18 point" and "bold" can both have different meanings in different fonts but, - except for very thin or unusual fonts, they should be sufficient. Since there are - so many different fonts, the general measures are used and a note regarding fancy - or thin fonts is included. + Resources). "18 point" and "bold" can both have different meanings in + different fonts but, except for very thin or unusual fonts, they should be sufficient. Since there + are so many different fonts, the general measures are used and a note regarding thin or unusual + fonts is included if the definition for large-scale text.

- -

The point size should be obtained from the user agent, or calculated based on font - metrics as the user agent does when evaluating this success criterion. Point sizes - are based on the CSS pt size as defined in - CSS3 Values. The ratio between sizes in points and CSS pixels is 1pt = 1.333px, therefore 14pt - and 18pt are equivalent to approximately 18.5px and 24px. -

- -

When fonts have anti-aliasing applied to make them look smoother, they can lose darkness - or lightness. Thus, the actual contrast can be reduced. Thicker stem widths will - reduce this effect (thin fonts could have the full stem lightened rather than just - the ends). Using larger fonts and testing for legibility in user agents with font - smoothing turned on is recommended. - +

When evaluating this Success Criterion, the font size in points should be obtained + from the user agent or calculated on font metrics in the way that user agents do. + Point sizes are based on the CSS pt size as defined in + CSS3 Values. The ratio between + sizes in points and CSS pixels is 1pt = 1.333px, therefore 14pt + and 18pt are equivalent to approximately 18.5px and 24px.

-

Because different image editing applications default to different pixel densities - (ex. 72 PPI or 96 PPI), specifying point sizes for fonts from within an image editing - application can be unreliable when it comes to presenting text at a specific size. + (e.g. 72ppi or 96ppi), specifying point sizes for fonts from within an + image editing application can be unreliable when it comes to presenting text at a specific size. When creating images of large-scale text, authors should ensure that the text in the resulting image is roughly equivalent to 1.2 and 1.5 em or to 120% or 150% of the - default size for body text as rendered by the browser. + default size for body text. For example, for a 72ppi image, an author would need + to use approximately 19pt and 24pt font sizes in order to successfully present images + of large-scale text to a user.

The 7:1 and 4.5:1 contrast ratios referenced in this Success Criterion are intended to be treated as threshold values. When comparing the computed contrast ratio to the Success Criterion ratio, the computed values should not be rounded (e.g. 4.499:1 would not meet the 4.5:1 threshold).

- +
+ +
+

Due to anti-aliasing, particularly thin or unusual fonts may be rendered by user agents with a much fainter + color than the actual text color defined in the underlying CSS. This can lead to situations where text has + a contrast ratio that nominally passes the Success Criterion, but has a much lower contrast in practice. + For these cases, we recommend as a best practice that authors choose a font with stronger/thicker lines, + or aim for a text color that exceeds the baseline requirements of this Success Criterion. +

-

The previously-mentioned contrast requirements for text also apply to - images of text (text that has been rendered into pixels and then stored in an image - format) as stated in Success Criterion 1.4.5 - - +

The contrast requirements for text also apply to images of text + (text that has been rendered into pixels and then stored in an image format) - see + Success Criterion 1.4.5: Images of Text.

This requirement applies to situations in which images of text were intended to be understood as text. Incidental text, such as in photographs that happen to include a street sign, are not included. Nor is text that for some reason is designed to be - invisible to all users. Stylized text, such as in corporate logos, should be treated + invisible to all viewers. Stylized text, such as in corporate logos, should be treated in terms of its function on the page, which may or may not warrant including the content in the text alternative. Corporate visual guidelines beyond logo and logotype are not included in the exception. @@ -96,15 +92,32 @@

Intent of Contrast (Enhanced)

in order to get a particular look.

-

Although this Success Criterion only applies to text, similar issues occur for content presented in charts, graphs, diagrams, and other non-text-based information which is covered by - Success Criterion 1.4.11 Non-Text Contrast. +

+ +

Images of text do not scale as well as text because they tend to pixelate. It is also + harder to change foreground and background contrast and color combinations for images + of text, which is necessary for some users. Therefore, we suggest using text wherever + possible, and when not, consider supplying an image of higher resolution. + +

+ +
+ +

The minimum contrast Success Criterion (1.4.3) applies to text in the page, including + placeholder text and text that is shown when a pointer is hovering over an object + or when an object has keyboard focus. If any of these are used in a page, the text + needs to provide sufficient contrast. +

+ +

Although this Success Criterion only applies to text, similar issues occur for content presented + in charts, graphs, diagrams, and other non-text-based information which is covered by + Success Criterion 1.4.11 Non-Text Contrast.

Rationale for the Ratios Chosen

-

A contrast ratio of 3:1 is the minimum level recommended by [[ISO-9241-3]] and [[ANSI-HFES-100-1988]] for standard text and vision. The 4.5:1 ratio is used in Success Criterion 1.4.3 to account for the loss in contrast that results from moderately low visual acuity, congenital or acquired color deficiencies, or the loss of contrast sensitivity that @@ -136,11 +149,22 @@

Rationale for the Ratios Chosen

[[ARDITI]].

+
+ +

Some people with cognitive disabilities require color combinations or hues that have + low contrast, and therefore we allow and encourage authors to provide mechanisms to + adjust the foreground and background colors of the content. Some of the combinations + that could be chosen may have contrast levels that will be lower than those found + in the Success Criteria. This is not a violation of this Success Criterion provided + there is a mechanism that will return to the default values set out here. +

+ +
+

The contrast ratio of 4.5:1 was chosen for level AA because it compensated for the loss in contrast sensitivity usually experienced by users with vision loss equivalent to approximately 20/40 vision. (20/40 calculates to approximately 4.5:1.) 20/40 is commonly reported as typical visual acuity of elders at roughly age 80. [[GITTINGS-FOZARD]] -

The contrast ratio of 7:1 was chosen for level AAA because it compensated for the @@ -154,9 +178,8 @@

Rationale for the Ratios Chosen

- -

Calculations in [[ISO-9241-3]] and [[ANSI-HFES-100-1988]] are for body text. A relaxed contrast ratio is provided for text that is much larger.

- +

Calculations in [[ISO-9241-3]] and [[ANSI-HFES-100-1988]] are for body text. A relaxed contrast + ratio is provided for text that is much larger.

@@ -167,10 +190,10 @@

Notes on formula

Conversion from nonlinear to linear RGB values is based on IEC/4WD 61966-2-1 [[IEC-4WD]].

-

The formula (L1/L2) for contrast is based on [[ISO-9241-3]] and [[ANSI-HFES-100-1988]] standards.

+

The formula (L1/L2) for contrast is based on [[ISO-9241-3]] and [[ANSI-HFES-100-1988]] standards.

The ANSI/HFS 100-1988 standard calls for the contribution from ambient light to be - included in the calculation of L1 and L2. The .05 value used is based on Typical Viewing + included in the calculation of L1 and L2. The .05 value used is based on Typical Viewing Flare from [[IEC-4WD]].

@@ -181,18 +204,14 @@

Notes on formula

-

- Refer to - related resources for a list of tools that utilize the contrast ratio to analyze the contrast of Web - content. - + Refer to + related resources for a list of tools that utilize the contrast ratio + to analyze the contrast of Web content.

-

See also 2.4.7: Focus Visible for techniques for indicating keyboard focus.

-
@@ -230,57 +249,39 @@

Resources for Contrast (Enhanced)

@@ -302,27 +303,21 @@

Situation A: text is less than 18 point if not bold and less than 14 point i
  1. - Ensuring that contrast of at least 10:1 exists between text and background behind the text -
  2. - Not specifying background color, not specifying text color, and not using technology features that change those defaults -
  3. - Providing a control with a sufficient contrast ratio that allows users to switch to a presentation that uses sufficient contrast -
@@ -336,27 +331,21 @@

Situation B: text is as least 18 point if not bold and at least 14 point if
  1. - Ensuring that a contrast ratio of at least 4.5:1 exists between text and background behind the text -
  2. - Not specifying background color, not specifying text color, and not using technology features that change those defaults -
  3. - Providing a control with a sufficient contrast ratio that allows users to switch to a presentation that uses sufficient contrast -
@@ -372,9 +361,7 @@

Additional Techniques (Advisory) for Contrast (Enhanced)

@@ -388,19 +375,15 @@

Failures for Contrast (Enhanced)