Which CSS selector is the most specific?

The ID selector is the most specific among the options provided. In CSS, specificity is a hierarchy that determines which styles are applied when multiple rules match the same element.

ID selectors target specific elements by their unique identifier, assigned to an element using the id attribute. This singular identification gives it a higher precedence than other selectors. For instance, if you have an element with an ID of "header," an ID selector like #header will directly and specifically style that element, regardless of other class or element selectors that may apply.

In contrast, class selectors, associated with the class attribute, can be reused across multiple elements, which decreases their specificity compared to ID selectors. Element selectors only apply styles to all instances of a specific element type, such as div or p, making them the least specific. The universal selector, indicated by the asterisk *, applies styles to all elements indiscriminately, resulting in the lowest specificity as it encompasses everything without targeting anything specifically.

Understanding this hierarchy is vital in CSS as it allows developers to manage which styles take precedence in the case of conflicting rules, assisting in creating a well-structured and predictable style sheet.