doc.html

<h1>Header</h1>

<h2 id="subheader">Subheader</h2>

<p>Paragraph</p>

<h2 id="subheader-2">Subheader 2</h2>

<h3 id="subsubheader-1">Subsubheader 1</h3>

<p>Subsub paragraph</p>

<p>Subsub paragraph 2</p>

<h3 id="subsubheader-2">Subsubheader 2</h3>

<p>Subsub paragraph 3</p>

<h2 id="subheader-3">Subheader 3</h2>

<p>Paragraph</p>
<pre><code>&lt;html&gt;code&lt;/html&gt;
</code></pre>
<p>Paragraph</p>

parse.rb

require "nokogiri"

html = File.read("./doc.html")
html_doc = Nokogiri::HTML(html)

headings = html_doc.xpath("//*[self::h1 or self::h2 or self::h3]")

headings.each do |h|
  elements_inside = []

  current_element = h.next
  while current_element && !current_element.name.include?("h")
    elements_inside << current_element
    current_element = current_element.next
  end

  puts "For header #{h.name} we found #{elements_inside.map(&:name)}"
end