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Viewport coordinates refer to the system used to define the position of elements within the visible area of a web page or application, known as the viewport. This coordinate system typically has its origin (0,0) at the top-left corner of the viewport. The x-axis extends horizontally to the right, and the y-axis extends vertically downwards. Values increase as you move right along the x-axis and down along the y-axis.

When discussing viewport coordinates, it's important to distinguish them from other coordinate systems, such as document coordinates or screen coordinates. Document coordinates consider the entire rendered document, including any parts currently scrolled out of view. Screen coordinates relate to the physical pixels of the display device. Viewport coordinates, however, are dynamic and change as the user resizes the browser window or scrolls the page.

In web development, viewport coordinates are commonly used in JavaScript to determine the position of an element relative to the visible window. Properties like `window.scrollX` and `window.scrollY` provide the scroll offset from the top-left of the document, and when combined with an element's offset from the document origin, can be used to calculate its position within the viewport. Many JavaScript libraries and frameworks abstract these calculations for convenience. Understanding viewport coordinates is crucial for implementing responsive designs, interactive elements, and features that rely on the user's current view.