CSS Specificity is the way the browser reads and determines which CSS value is used first. It works by determining the weight of the CSS selector.
The contrast ratio is the measurement between colors and background colors on a web page. This is important to make sure it meets requirements because it can become a legal issue if the contrast doesn't meet the minimum requirements.
Some of the reasons why you must meet the requirements are they make it easier for everyone who is looking at your web page and can help people who have disabilities. Then the other big issue is that it becomes a legal issue and you could get fined for not meeting the standards.
The group that creates the rules is called AG WG, which stands for Accessibility Guidelines Working Group.
Overall it is important to understand the order and the weight of CSS properties so you can understand which will come first. Then it is also important to understand the proper contrast ratios so that you can avoid dealing with any legal problems and make your web pages more enjoyable to individuals who visit.