A little SAX parser solution for: https://twitter.com/keystonelemur/status/1003752494797737984

fox_handler.rb

# frozen_string_literal: true

require 'open-uri'
require 'ox'

class FoxHandler < Ox::Sax
  def initialize
    @h_tags = []
    @h = nil
  end

  def start_element name
    return unless name.match? /\Ah\d+\z/i

    @h = name[1..-1].to_i
    if @h_tags.last.to_i < @h
      @h_tags << @h
    elsif @h_tags.last > @h
      @h_tags.pop
    end

    print '  ' * @h_tags.size.pred
    print "[#{name}] "
  end

  def end_element _
    @h = nil
  end

  def text value
    return unless @h
    puts value
  end
end

html = URI.parse('https://jquery.com').read

def header_hierarchy html
  Ox.sax_parse FoxHandler.new, html
end

header_hierarchy html
#[h2] jQuery
#  [h3] Lightweight Footprint
#  [h3] CSS3 Compliant
#  [h3] Cross-Browser
#[h2] What is jQuery?
#[h2] Other Related Projects
#  [h3] Resources
#[h2] A Brief Look
#  [h3] DOM Traversal and Manipulation
#  [h3] Event Handling
#  [h3] Ajax
#  [h3] Books