viewport指代计算机图形学上的多边形(通常是矩形)，代表当前被viewed的区域，即当前位于用户浏览器内处于visible状态的区域。Viewport之外的区域处于不可见状态，直至其被scroll到viewport内为止。
A viewport represents a polygonal (normally rectangular) area in computer graphics that is currently being viewed. In web browser terms, it refers to the part of the document you’re viewing which is currently visible in its window (or the screen, if the document is being viewed in full screen mode). Content outside the viewport is not visible onscreen until scrolled into view.
参考文献
Deferred loading is a technique that is used to postpone the rendering of CMS components. Any component that is outside the viewport (that is, the area that is visible to the user) is not rendered in advance.
