CSS pseudo-elements act as "phantom" nodes that allow developers to style specific fragments of a document, such as the first line of a paragraph or user text selections, without altering the HTML structure. Unlike pseudo-classes that target an element’s state, these synthetic elements can inject generated content using the ::before and ::after keywords or modify internal UI parts like input placeholders and list markers. Modern browser engines like Blink implement these by creating internal objects in the layout tree rather than adding real nodes to the DOM. While these tools offer powerful typographic and decorative control, they come with property restrictions to maintain layout stability and require careful use to ensure web accessibility. The technology has evolved from simple typographic aids to a complex system supporting Web Components through specialized selectors like ::part and ::slotted. Recent developments also include the CSS Highlight API and View Transitions, which expand the capability of pseudo-elements into dynamic application states and custom text styling.