The root element in HTML and XHTML

From the markup point of view, the root element of HTML (and XHTML) documents is the html element, consisting of the entire document (excluding only the document type declaration, DOCTYPE declaration). That element always exists in a document, even if the start and end tags <html> and </html> (which are optional in “classic” HTML) have been omitted.

From the rendering point of view, the situation is more complex. root element of HTML (and XHTML)

In this mode, browsers do their utmost to follow the CSS specification (as well as XHTML and others) without regard for legacy behaviors: the body element is only as tall as its content.

For example, if you set
html { background: yellow; color: black; } body { background: aqua; color: black; }

That's because the root element covers the viewport, which is what I suspect your want ("page" is an ambiguous term). In XHTML, 'body' is not the root element, and so does not cover the viewport. It may appear to for long documents, but that's an illusion: it's actually longer than the viewport in those cases.

This document is declared with an HTML 4.01 document type: