Document.documentElement returns the Element that is the root element of the document (for example, the <html> element for HTML documents).
Syntax
var element = document.documentElement;
Example
var rootElement = document.documentElement;
var firstTier = rootElement.childNodes;
// firstTier is the NodeList of the direct children of the root element
for (var i = 0; i < firstTier.length; i++) {
   // do something with each direct kid of the root element
   // as firstTier[i]
}
Notes
HTML documents typically contain a single child node, <html>, perhaps with a DOCTYPE declaration before it. XML documents often contain multiple child nodes: the root element, the DOCTYPE declaration, and  processing instructions.
That's why you should use document.documentElement rather than document.firstChild to get the root element.
Specifications
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