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The visible area of a web page on your screen — like the window frame that determines how much of a painting you can see at once.
The rectangular area in which the browser renders content. The <meta name='viewport'> tag controls initial scale and width on mobile devices, and viewport units (vw, vh) let you size elements relative to it.
CSS defines three viewport variants — small (svw/svh), large (lvw/lvh), and dynamic (dvw/dvh) — to address mobile browser chrome that appears and disappears. The visual viewport (what the user actually sees) can differ from the layout viewport when pinch-zoomed, with the Visual Viewport API exposing offsetLeft, offsetTop, and scale properties.