The read-only scrollY property of the Window interface returns the number of pixels by which the document is currently scrolled vertically. This value is subpixel precise in modern browsers, meaning that it isn't necessarily a whole number. You can get the number of pixels the document is scrolled horizontally from the scrollX property.| MDN Web Docs
In computer graphics, a viewport represents a polygonal (normally rectangular) area that is currently being viewed.| MDN Web Docs
Baseline identifies the availability of web platform features across popular browsers, including APIs, CSS properties, and JavaScript syntax. Baseline describes web features as being either widely available or newly available. Features that do not meet the Baseline criteria are said to have limited availability.| MDN Web Docs
1. Background| drafts.csswg.org
