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Living Examples - Conveyed with Color |
(c) Web pages shall be designed so that all information conveyed with color is also available without color, for example from context or markup.
Color is often used to make certain words or passages stand out (e.g. a health alert might be written in red font).
If color is used to highlight the importance of specific content, the content should be written in such a way that it conveys its importance without the
use of color. This will benefit those who require assistive technology to access information from a user agent.
Color-blind people can easily be accommodated by using simple markup, such as the <em> or <strong> tags. Some assistive technologies may also be able to
interpret these tags.
It is important to have a strong color contrast between hyperlinked text and content text, in particular for sites that utilize style sheets that don't
underline hyperlinks.
Although usability studies have indicated that the original choices for hyperlink colors were unfortunate, people have ultimately learned to associate blue
with "unvisited link" and purple with "previously visited link," to the point where sites that use other colors for their links cause some level of confusion.
Although that in itself is more of a usability issue, take into consideration the users who have preset their default Style Sheet so that all pages appear
with the font size and color that is best suited for their visual abilities. Developers who use Style Sheets to define their site's appearance must not
allow the page to override a user's default Style Sheet. This same practice will, in turn, ensure that the browser's defined colors will be rendered for
all other users.
Also, on pages where the author overuses underlined words and phrases, hyperlinks that are allowed to display the browser's default colors will provide
adequate contrast for those with color-impaired vision.
In accounting, it is common to show negative monetary values with red font, and positive values in black font.
Users who require assistive technology will not know what color the numeric value is displayed in, so they won't know whether the number is a positive or
negative value. Consider the practice of enclosing negative values within parenthesis ( ), or simply including the negative sign.
Another common practice is to use different colors to show values for different categories or groupings (e.g. government employee work hours are shown in
red, contractor work hours are shown in blue, and total work hours are shown in black). In such a case, you must label the numeric values with the name
of the category or grouping which it represents, so that those who use assistive technology will know which category goes with which numeric value (e.g.
a properly marked up table will be read by assistive technology as: Government workers: 135.5; Contractors: 346.0; total: 481.5).
Numerical groupings that are displayed within tables can be labeled using <th id=""> and <td headers="">, as described in the
Tables guidelines.
Numerical groupings that are displayed without the use of tables should contain text that clearly states their meaning.
Text displayed in red (or some other color) is often used to denote a required field on an electronic form.
Both color-blind users and those who require assistive technology could miss the fact that a particular field is required. Use a special character, such
as an asterisk (*), to show required fields. Also, mention at the top of the form what special character represents the required field indicator.
Another method of complying with this checkpoint is to include the word "required" as part of the label for that field so that users of assistive technology
will hear it. A variation of this method would be to place a graphical image of the special character that identifies the field as required, then add the
value "required field" to the <img> tag's alt attribute.
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Curator:
Responsible NASA Official:
Last Updated: March 6, 2009
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