Any technology
This failure relates to:
This failure occurs when people with low vision are not able to read text that is displayed over a background image. When there is not sufficient contrast between the background image and the text, features of the background image can be confused with the text making it difficult to accurately read the text.
To satisfy Success Criterion 1.4.3 and 1.4.6, there must be sufficient contrast between the text and its background. For pictures, this means that there would need to be sufficient contrast between the text and those parts of the image that are most like the text and behind the text.
Black text overlays an image with black lines. The lines cross behind the letters making F's look like E's etc.
Black text overlays an image with with dark gray areas. Wherever the text crosses a dark gray area the contrast is so bad that the text cannot be read.
Quickcheck: First do a quick check to see if the contrast between the text and the area of the image that is darkest (for dark text) or lightest (for light text) meets or exceeds that required by the Success Criterion (1.4.3 or 1.4.5). If the contrast meets or exceeds the specified contrast, then there is no failure.
If the Quickcheck is false, then check to see if the background behind each letter has sufficient contrast with the letter.
If Quickcheck is false and #2 is false as well then this failure condition applies and the content fails the contrast Success Criterion.
Techniques are informative—that means they are not required. The basis for determining conformance to WCAG 2.0 is the success criteria from the WCAG 2.0 standard—not the techniques. For important information about techniques, please see the Understanding Techniques for WCAG Success Criteria section of Understanding WCAG 2.0.