Fixed positioning removes an element from the normal document flow and positions it relative to the viewport. When position: fixed is applied, the element maintains its position on screen regardless of scrolling, making it appear "stuck" in place as the user scrolls. This is commonly used for navigation bars, floating action buttons, or persistent headers that need to remain visible throughout the user's browsing experience.

B. Relative: Elements remain in normal flow and move with page scroll.

C. Absolute: Positioned relative to nearest positioned ancestor, scrolls with page.

D. Static: Default positioning, elements scroll normally with document flow.

1. MDN Web Docs, "CSS Position Property", Mozilla Developer Network, Section: "Fixed positioning", https://developer.mozilla.org/en-US/docs/Web/CSS/position#fixed

2. W3C CSS Positioned Layout Module Level 3, "6.5. Fixed positioning", W3C Working Draft, https://www.w3.org/TR/css-position-3/#fixed-pos

3. "CSS: The Definitive Guide, 4th Edition" by Eric Meyer & Estelle Weyl, O'Reilly Media, 2017, Chapter 11: "Positioning", pp. 542-545

4. "CSS Secrets" by Lea Verou, O'Reilly Media, 2015, Chapter 2: "Backgrounds & Borders", Section on Fixed Backgrounds, pp. 48-50

5. Stanford CS142: Web Applications Course Materials, "CSS Layout and Positioning", Lecture 4 Slides, pp. 22-24, https://web.stanford.edu/class/cs142/