diff --git a/Rakefile b/Rakefile
index 5e9d20c..e2aaac3 100644
--- a/Rakefile
+++ b/Rakefile
@@ -115,7 +115,7 @@ namespace :spec do
   RSpec::Core::RakeTask.new(:all) do |t|
     t.verbose = false
     t.pattern = 'spec/*_spec.rb'
-    t.rspec_opts = '-f documentation'
+    t.rspec_opts = '-f documentation -w'
   end
 end
 
diff --git a/lib/html/mixin/html_handler.rb b/lib/html/mixin/html_handler.rb
index 47868bf..17b19b0 100644
--- a/lib/html/mixin/html_handler.rb
+++ b/lib/html/mixin/html_handler.rb
@@ -14,11 +14,11 @@ def modify_html(attribute, arg = nil)
         if @html_begin.scan(/\b#{attribute}\b/).empty?
           case arg
             when Integer
-              @html_begin << " #{attribute}=#{arg}"
+              @html_begin += " #{attribute}=#{arg}"
             when TrueClass
-              @html_begin << " #{attribute}"
+              @html_begin += " #{attribute}"
             else
-              @html_begin << " #{attribute}='#{arg}'"
+              @html_begin += " #{attribute}='#{arg}'"
           end
         else
           case arg
@@ -41,8 +41,8 @@ def modify_html(attribute, arg = nil)
       #
       def html(formatting = true)
         if HTML::Table.html_case == 'upper'
-          @html_begin.upcase!
-          @html_end.upcase!
+          @html_begin = @html_begin.upcase
+          @html_end = @html_end.upcase
         end
 
         ilevel = 0
@@ -57,12 +57,12 @@ def html(formatting = true)
 
         if is_a?(Array)
           if formatting
-            html << map { |e| "\n#{e.html(formatting)}" }.join
+            html += map { |e| "\n#{e.html(formatting)}" }.join
           else
-            html << map { |e| e.html(formatting).to_s }.join
+            html += map { |e| e.html(formatting).to_s }.join
           end
         else
-          html << @html_body
+          html += @html_body
         end
 
         #####################################################################
@@ -78,37 +78,37 @@ def html(formatting = true)
             if self.class.respond_to?(:end_tags?)
               if formatting
                 if self.class.end_tags?
-                  html << ("\n" << (' ' * ilevel) << @html_end)
+                  html += ("\n" + (' ' * ilevel) + @html_end)
                 end
               else
-                html << ((' ' * ilevel) << @html_end) if self.class.end_tags?
+                html += ((' ' * ilevel) + @html_end) if self.class.end_tags?
               end
             else
               if formatting
-                html << ("\n" << (' ' * ilevel) << @html_end)
+                html += ("\n" + (' ' * ilevel) + @html_end)
               else
-                html << ((' ' * ilevel) << @html_end)
+                html += ((' ' * ilevel) + @html_end)
               end
             end
           else
             unless self.class.respond_to?(:end_tags?)
               if formatting
-                html << ("\n" << (' ' * ilevel) << @html_end)
+                html += ("\n" + (' ' * ilevel) + @html_end)
               else
-                html << ((' ' * ilevel) << @html_end)
+                html += ((' ' * ilevel) + @html_end)
               end
             end
           end
         else
           if HTML::Table.global_end_tags?
             if self.class.respond_to?(:end_tags?)
-              html << @html_end if self.class.end_tags?
+              html += @html_end if self.class.end_tags?
             else
-              html << @html_end
+              html += @html_end
             end
           else
             unless self.class.respond_to?(:end_tags?)
-              html << @html_end
+              html += @html_end
             end
           end
         end
diff --git a/lib/html/table.rb b/lib/html/table.rb
index 54ec612..ae551d4 100644
--- a/lib/html/table.rb
+++ b/lib/html/table.rb
@@ -169,8 +169,7 @@ def self.html_case
     #
     def self.html_case=(arg)
       expect(arg, String)
-      arg.downcase!
-      unless %w[upper lower].include?(arg)
+      unless %w[upper lower].include?(arg.downcase)
         msg = "Argument to html_case() must be 'upper' or 'lower'"
         raise ArgumentError, msg
       end