diff --git a/lib/oga.rb b/lib/oga.rb
index d671571..8eee85b 100644
--- a/lib/oga.rb
+++ b/lib/oga.rb
@@ -7,6 +7,7 @@ require_relative 'oga/version'
 require_relative 'oga/oga'
 require_relative 'oga/lru'
 require_relative 'oga/entity_decoder'
+require_relative 'oga/node_name_set'
 
 # Load these first so that the native extensions don't have to define the
 # Oga::XML namespace.
diff --git a/lib/oga/node_name_set.rb b/lib/oga/node_name_set.rb
new file mode 100644
index 0000000..3d0c57d
--- /dev/null
+++ b/lib/oga/node_name_set.rb
@@ -0,0 +1,16 @@
+module Oga
+  ##
+  # Class for storing (HTML) element names in a set and automatically adding
+  # their uppercase equivalents.
+  #
+  class NodeNameSet < Set
+    ##
+    # @param [Array] values
+    #
+    def initialize(values = [])
+      values = values + values.map(&:upcase)
+
+      super(values)
+    end
+  end # NodeNameSet
+end # Oga
diff --git a/lib/oga/xml/html_void_elements.rb b/lib/oga/xml/html_void_elements.rb
index 2efbb6f..c0ee4d1 100644
--- a/lib/oga/xml/html_void_elements.rb
+++ b/lib/oga/xml/html_void_elements.rb
@@ -4,9 +4,9 @@ module Oga
     # Names of the HTML void elements that should be handled when HTML lexing
     # is enabled.
     #
-    # @return [Set]
+    # @return [Oga::NodeNameSet]
     #
-    HTML_VOID_ELEMENTS = Set.new([
+    HTML_VOID_ELEMENTS = NodeNameSet.new([
       'area',
       'base',
       'br',
@@ -24,7 +24,5 @@ module Oga
       'track',
       'wbr'
     ])
-
-    HTML_VOID_ELEMENTS.merge(HTML_VOID_ELEMENTS.map { |name| name.upcase })
   end # XML
 end # Oga
diff --git a/spec/oga/node_name_set_spec.rb b/spec/oga/node_name_set_spec.rb
new file mode 100644
index 0000000..ee0c374
--- /dev/null
+++ b/spec/oga/node_name_set_spec.rb
@@ -0,0 +1,11 @@
+require 'spec_helper'
+
+describe Oga::NodeNameSet do
+  describe '#initialize' do
+    it 'adds uppercase equivalents of the input strings' do
+      set = described_class.new(%w{foo bar})
+
+      set.to_a.should == %w{foo bar FOO BAR}
+    end
+  end
+end