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HTML/CSSIntermediate#css#selectors
HTML/CSSBeginner#css#selectors
What is the difference between pseudo-classes and pseudo-elements?
Pseudo-classes (single colon, like :hover, :nth-child, :focus) select elements based on state or position. Pseudo-elements (double colon, like ::before, ::after, ::first-line) create or target a sub-part of an element that doesn't exist as a separate DOM node.
Example
a:hover { color: red; }
li:nth-child(odd) { background: #f5f5f5; }
.quote::before { content: '\201C'; }