This took me quite a bit of research to wrap my head around, and it will still require additional practice, but I think I have the overall concepts.
Specificity is a way the browser (user-agent) ranks the CSS code to determine which rule takes precidence. It's a numeric value given for specific code within the CSS. In CSS Specificity, the largest numerical value takes precidence over the lower values. The assigned values are as follows :
Value of 1000 =Inline Styles
Value of 100 = IDs
Value of 10 = Class, Attribute, &Pseudo-Class
Value of 1 = Elements & Pseudo Elments
Although the browser does the calculations, it is important for web designers to understand how these calculations work for building, maintaining, ordering & refreshing webpages. In this way, it can make the workflow less stressful and more efficiant.
The contrast ratio of a site is essentially the measurement between one color and another color. The lower the number, the less contrast. There are many tools on the web that can help a designer find the contrast ratio of their site. Even the developer tools in the browsers have to ablilty to find this calculation.
Whenever using color in images, the colors should have a contrast ratio of at least 4.5. This way the text is readable to those who may have color blindness or other eye health conditions. If the contrast ratios are not high enough, an organization could be sued; this is especially serious for institutions who receive funding through government programs.
The Accessible Platform Architectures (APA) Working Group ensures W3C specifications provide support for accessibility to people with disabilities. The group's expertise about the needs of users with disabilities is provided to the W3C to ensure that accessiblity measures are in place. They serve as both a resource and provide education to the W3C community.
Specificity is a key componant of CSS, so understanding how it works will save time and frustration when troubleshooting. Accessibility opens up website use to all people, everywhere, which has become a fundamental necessity in this modern world. Understanding specificity and careful adhearance to website accessiblity rules are necessary for successful web design.