That's not a "first-page".
<html>
and <body>
are set to 100% height, and then its content is set to overflow out of the body.
The first child in the list of children has a height of at least 100% (which is 100% of the parent-height, which is 100% of the browser height).
The rest of the children have auto-defined heights based on their contents.
There are several ways of getting this end result, using position:fixed or position:absolute, et cetera.