Test for WCAG success criterion 1.4.3: Contrast

[hannolans]

http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
The visual presentation of text and images of text has a contrast ratio of at least 4.5:1

Applicability

Always

Selector

Text parts:

• Text is less than 18 point if not bold and less than 14 point if bold (Situation A)
• Text is at least 18 point if not bold and at least 14 point if bold (Situation B)

No minimum contrast requirement for text or images of text:

• that are part of an inactive user interface component
• that are pure decoration
• that are not visible to anyone,
• that are part of a picture that contains significant other visual content
• that is part of a logo or brand name

Failures

• F24: Specifying foreground colors without specifying background colors or vice versa (Test for WCAG failure F24: specifying foreground colors without specifying background colors or vice versa #187 )
• F83: Using background images that do not provide sufficient contrast with foreground text (or images of text)

Techniques

• G148: Not specifying background color, not specifying text color, and not using technology features that change those defaults (see F24 Test for WCAG failure F24: specifying foreground colors without specifying background colors or vice versa #187)
• G174: Providing a control with a sufficient contrast ratio that allows users to switch to a presentation that uses sufficient contrast
• G18: Ensuring that a contrast ratio of at least 4.5:1 exists between text (and images of text) and background behind the text for situation A AND G145: Ensuring that a contrast ratio of at least 3:1 exists between text (and images of text) and background behind the text for situation B ( Test for WCAG technique G145, G18 and G17: Ensuring that a contrast ratio exists between text (and images of text) and background behind the text #71 )

Relationship between techniques and Failures

• G18 and G145 are exactly the same, only the ratio differs
• G18 is also used on AAA level, but then for situation B.
• F24 is counterpart for G148
• F83 is part of the check in G18+G145, but as images can't change their contrast with a control or backgorund css it's a real failure and G174 or G148 can't help that.
• G18 + G145 is the most common technology, but if they fail, we can't fail this success criterion if we don't check for G148 (easy) or G174 (difficult as we should detect a control)

[attiks]

PR created for checking font size and weight to determine ratio: #216

[jessebeach]

Working in branch issue-129