gistfile1.txt

Selector: dd.foo
<dd class="foo">Value</dd>
<dd class="foo">Trying to select this dd</dd>

Selector: dd:last
<dd class="other">Other Value</dd>

Selector: dd.foo:last-child

Selector: dd.foo:last-of-type

Selector: dd.foo:nth-of-type(3)
<dd class="foo">Trying to select this dd</dd>

Selector: dd.foo:nth-last-of-type(1)
<dd class="foo">Trying to select this dd</dd>

Selector: dd.foo:nth-last-child(1)
<dd class="foo">Trying to select this dd</dd>

nokogiri_fail.rb

require 'nokogiri'

doc = <<-DOC
  <dl>
    <dt>Label</dd>
    <dd class=other>Other Value</dd>

    <dt>Label</dt>
    <dd class=foo>Value</dd>
    <dd class=foo>Trying to select this dd</dd>
    <dd class=other>Other Value</dd>
  </dl>
DOC

n = Nokogiri::HTML(doc)

[
  "dd.foo",
  "dd:last",
  "dd.foo:last-child",
  "dd.foo:last-of-type",
  "dd.foo:nth-of-type(3)",
  "dd.foo:nth-last-of-type(1)",
  "dd.foo:nth-last-child(1)",
].each do |selector|
  puts "Selector: #{selector}"
  puts n.css(selector)
  puts "\n"
end