Mastering CSS Selectors: Understanding Specificity and Best Practices

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.

W3C Specification for CSS Selectors Level 4

To apply CSS to an element you need to select it. CSS provides you with a number of different ways to do this, and you can explore them in this module.