The <html> tag is not required.
From the DTD:
<!ELEMENT HTML O O (%html.content;) -- document root element -->
The two Os indicate that the start and end tags (respectively) are optional.
The element, on the other hand, is required (but the language is designed so that browsers can imply it).
Since a DOM consists of a tree of nodes, you have to have one node (the root element) for everything else to hang from, and that is the html element.
It is also a really useful place to stick a lang attribute that will apply to the entire document.
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