This technique relates to 1.4.1: Use of Color (Failure).
This failure applies to any technology.
The objective of this failure is to avoid situations in which people who cannot perceive color differences cannot identify links (when people with color vision can identify links). Link underlines or some other non-color visual distinction are required (when the links are discernible to those with color vision).
While some links may be visually evident from page design and context, such as navigational links, links within text are often visually understood only from their own display attributes. Removing the underline and leaving only the color difference for such links would be a failure because there would be no other visual indication (besides color) that it is a link.
Note
Red and pink are the same color (hue) but they have different lightness (which is not color). So red and pink would pass the requirement for "not distinguished by color (hue) alone" since they differ by lightness (which is not color) - as long as the difference in lightness (contrast) is 3:1 or greater. For example, if surrounding text is red and the link is pink it would pass. Similarly a light green and a dark red differ both by color and by lightness so they would pass if the contrast (lightness) difference is 3:1 or greater before focus or pointing.
There is no requirement that links be identifiable by people who cannot perceive color if they are not perceivable for those with color vision. (e.g. if the links are hidden for everyone – as in a game or test).
If the non-color cue only happens when the mouse hovers over the link or when the link receives focus, it is still a failure.
If the link is a different color and bold it would not fail because the boldness is not color dependent.
A Web page includes a large number of links within the body text. The links are styled so that they do not have underlines and are very similar in color to the body text. This would be a failure because users would be unable to differentiate the links from the body text.
<head>
<style>
p a:link {text-decoration: none}
p a:visited {text-decoration: none}
p a:active {text-decoration: none}
p a:hover {text-decoration: underline; color: red;}
</style>
</head>
<body>
<p>There are many resources to find out more about the
<a href="rain-in-spain.html">rain in Spain</a>.
</p>
</body>Note
If the visual cue is only provided on hover (as in the example above), it would still fail.