How come selectors in $.fn.find() cannot reference the tree beyond the current $ scope?

Question 1

Your first selector $('html head') works just like find - gets any <head> descendants of HTML

HTML──┐  start here and find head
    HEAD

Your second selector $('html').find('head') gets html element - then finds all descendant <head> elements

HTML──┐ start here and find head
    HEAD

Your third selector $('html').find('head').filter('html head') gets html element - then finds all descendant <head> elements - the filter is really pointless because the returned head elements are guaranteed to be descendants of html because of this $('html').find('head')

HTML──┐  start here and find head
    HEAD

Your lastone that isn't working $('html').find('html head') is searching for structure below

HTML──┐  start here and find "html head" 
    HTML──┐    
         HEAD
but really your structure is like this
HTML──┐  
    HEAD

which html doesn't have a descendant html element so that results in 0 elements returned

If you want to keep the HTML element in the collection you can use .addBack()

$('html') // get html element
  .find('html head') // find head element
  .addBack() // add back the html element
  // if jQuery 1.7 and lower use .andSelf()
  .length  // this will result in 1 element - the HTML element

Question 2

Because there is no <html> tag nested inside <html> tag

But:

$('html').find('html, head').length // return 1 here coz comma means 'or'

Question 3

Per the documentation:

Find: Get the descendants of each element in the current set of matched elements, filtered by a selector, jQuery object, or element.

HTML is not a descendant of HTML

http://api.jquery.com/find/