Add Nokogiri::XML::DTD#html_dtd? and #html5_dtd?.
Simplify meta_encoding and meta_content_type (private).
Fix a typo luckily caused no problem.
Add Nokogiri::XML::Node#prepend_child. #664
Put <title> in <head> if it exists.
Prefer XPath queries to CSS selectors for performance.
Make Nokogiri::HTML::Document#meta_encoding= add a meta element if not present.
Add a note that libxml2 automatically inserts a meta tag in <head>.