The understanding of the DOM is still very shallow, HTMLDocument is supposed to be a constructor, according to the prototype inheritance, these are no problem:

 HTMLDocument. Constructor==Function//true 
HTMLDocument. Prototype==document. __proto__//true 
HTMLDocument==document. The constructor//true 

Why this is false?

 HTMLDocument. __proto__==Function. The prototype//false 

After all, the structure of the general Function of the prototype are pointing in the direction of the Function. The prototype of:

 function Foo () {}; 
Foo __proto__==Function. The prototype;//true