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

output

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

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

Selector: dd.criterion:last

Selector: dd.criterion:last-of-type

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

Selector: dd[class=criterion]:last