To be pedantic, it's the tags that are optional, not the elements. The html, header and body elements are still there even if you don't write the tags.
Quoting from the document you linked:
"Omitting an element's start tag in the situations described below does not mean the element is not present; it is implied, but it is still there. For example, an HTML document always has a root html element, even if the string <html> doesn't appear anywhere in the markup."
Quoting from the document you linked:
"Omitting an element's start tag in the situations described below does not mean the element is not present; it is implied, but it is still there. For example, an HTML document always has a root html element, even if the string <html> doesn't appear anywhere in the markup."