The reason is not to refer to content or functionality only by color. For example, references to an internal or external link to the book that would be reported by the only blue color will not be readable by some audiences.
Objectives
Allow access to information for users including terminal or reading software technical assistance or disability (such as colour color) do not allow to visualize or differentiate colors
Improve accessibility of content to readers with disabilities
Implementation
Provide a complete colour to convey the information it wears. This complementary, independent of the CSS formatting layer, can be multiple orders, e.g. :
Prevent semantic markup (strong, em, etc. ;
Add axes, patterns, borders, etc. in maps and charts.
Control
Verification requires visually comparing two types of display of book : normal display and color display will be disabled (rendered to computer screen and rendered to smooth with greyscale screen).
Verification requires visually comparing two types of display of book : normal display and color display will be disabled (rendered to computer screen and rendered to smooth with greyscale screen).