Understanding CSS specificity is fundamentally important for web developers. When writing two competing rules in CSS, the developer must know which is going to overrule the other. To calculate specificity, browsers use a number system (0, 0, 0). The right-most digit counts the number of elements in a CSS selector. The second digit counts the number of classes selected, and the first digit specifies how many ids are selected. If a CSS selector included 1 id, 2 classes, and 3 elements, the specificity number would be 123. The browser will always use the higher number to overwrite competing rules. Ordering link pseudo-classes is important because they all have the same specificity, so whichever is listed last will be used. Visited link style must overwrite the general link style, because otherwise it would not appear. The same goes for hover, focus, and active. Each must overwrite the previous.
When using colored text against a colored background, it is important to make sure that the contrast ratio is high enough. The contrast ratio is a measure of the difference between the two colors, and it must be high enough to be readable by a wide variety of web users. The standard for contrast ratio requires at least a 4:5:1 ratio for normal text and 3:1 for large text.
Understanding contrast ratio and developing appropriate ratios is critical on the web because it is the law. The rules were created so that websites are accessible to all users, and developers must take these standards seriously. It is important that all users can access information on the web, including those with color vision deficit. This criterion was created to ensure color is not a key factor in consuming information from the web and there is adequate contrast between text and the background.
The WCAG (Web Content Accessibility Guidelines) are created by the W3C and are the international standard for websites to make them more accessible to people with disabilities. These guidelines are developeed by the Accessibility Guidelines Working Group (AG WG) which is part of the W3C. They create the standards for contrast ratios and beyond to ensure everyone can use the web.
It is critical that everyone in the world--including those with disabilities--have access to the revolutionary technology that is the web. Developers must abide by the accessibility standards: not only because it is the law, but because it is the right thing to do. These standards ensure all web pages are available and usable for anyone and everyone.