Specificity is the algorithm used by browsers to determine the CSS declaration that is the most relevant to an element, which in turn, determines the property value to apply to the element. The specificity algorithm calculates the weight of a CSS selector to determine which rule from competing CSS declarations gets applied to an element.
The color contrast between background and foreground content should be great enough to ensure legibility.
The WCAG guidelines recommends these ratios for type of content:You must meet contrast ratios to benefit all users. It is especially beneficial to users with certain types of color blindness and similar conditions who experience low contrast and have trouble differentiating between similar colors and details such as edges and borders.
The WCAG technical documents are developed by the Accessibility Guidelines Working Group (AG WG) (formerly the Web Content Accessibility Guidelines Working Group), which is part of the World Wide Web Consortium (W3C) Web Accessibility Initiative (WAI).
CSS Specification defines how CSS properties should be implemented by browser vendors along with detailed algorithms, code samples and tabular information. Making good use of contrast ratios and understanding why you must meet them will benefit all users in the long run. Having good accessibility practices will make it easier for all users to be able to use your site.