MDN :
The Document.documentElement read-only property returns the Element that is the root element of the document (for example, the <html> element for HTML documents) .
层级关系：
1. document
2. documentElement(html)
3. head, body
MDN :
The Document.documentElement read-only property returns the Element that is the root element of the document (for example, the <html> element for HTML documents) .
层级关系：
1. document
2. documentElement(html)
3. head, body