Specificity Explained
Specificity is an ordering system used by browsers when reading your markup that puts a value on styles. This value determines which property values or assigned styles will be shown and which will be overrode. It uses an algorith assigning the value to things such as ID's, classes, and element selectors.
Contrast Ratios Explained
Contrast ratios are used to determine the visability of text against back colors being used. The higher the contrast ratio the easier it will be to read the text.
Contrast Ratios - Why?
Contrast ratios are so important because of accessability. Visually challened either through poor eyesight or difficulties with color associations can have a difficult time discerning between colors if they do not meet a certain contrast ration.
W3C Accessibility Rules - Who?
The W3C Web Accessibility Initiative brings together people from industry, disability organizations, government, and research labs from around the world to develop the guidelines and resources to help make the Web accessible.
Summary
There are many ways that you can style and manipulate buttons and many things to consider when building designs to make sure they work well for all audiences.