diff --git a/concourse/tasks/rake-test/run.sh b/concourse/tasks/rake-test/run.sh
index 8b751200..9dc1103e 100755
--- a/concourse/tasks/rake-test/run.sh
+++ b/concourse/tasks/rake-test/run.sh
@@ -4,7 +4,9 @@ set -e -x -u
 
 pushd loofah
 
+  # TODO: remove this once fefc629 (2019-11-25) is far enough in the past
   export RUBYOPT="--enable-frozen-string-literal --debug=frozen-string-literal"
+
   bundle install
   bundle exec rake test
 
diff --git a/lib/loofah.rb b/lib/loofah.rb
index a25dbf9f..37338efd 100644
--- a/lib/loofah.rb
+++ b/lib/loofah.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 $LOAD_PATH.unshift(File.expand_path(File.dirname(__FILE__))) unless $LOAD_PATH.include?(File.expand_path(File.dirname(__FILE__)))
 
 require "nokogiri"
diff --git a/lib/loofah/elements.rb b/lib/loofah/elements.rb
index 6e76c660..abfbe33b 100644
--- a/lib/loofah/elements.rb
+++ b/lib/loofah/elements.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'set'
 
 module Loofah
diff --git a/lib/loofah/helpers.rb b/lib/loofah/helpers.rb
index 4cdfa3a3..d8928ded 100644
--- a/lib/loofah/helpers.rb
+++ b/lib/loofah/helpers.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Loofah
   module Helpers
     class << self
diff --git a/lib/loofah/html/document.rb b/lib/loofah/html/document.rb
index 9eef099f..8b887e3e 100644
--- a/lib/loofah/html/document.rb
+++ b/lib/loofah/html/document.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Loofah
   module HTML # :nodoc:
     #
diff --git a/lib/loofah/html/document_fragment.rb b/lib/loofah/html/document_fragment.rb
index 25f16b56..359dffc8 100644
--- a/lib/loofah/html/document_fragment.rb
+++ b/lib/loofah/html/document_fragment.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Loofah
   module HTML # :nodoc:
     #
diff --git a/lib/loofah/html5/libxml2_workarounds.rb b/lib/loofah/html5/libxml2_workarounds.rb
index fc3fb244..a867cdd8 100644
--- a/lib/loofah/html5/libxml2_workarounds.rb
+++ b/lib/loofah/html5/libxml2_workarounds.rb
@@ -1,4 +1,5 @@
 # coding: utf-8
+# frozen_string_literal: true
 require 'set'
 
 module Loofah
diff --git a/lib/loofah/html5/safelist.rb b/lib/loofah/html5/safelist.rb
index f9f423c7..f0a0dcbc 100644
--- a/lib/loofah/html5/safelist.rb
+++ b/lib/loofah/html5/safelist.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "set"
 
 module Loofah
diff --git a/lib/loofah/html5/scrub.rb b/lib/loofah/html5/scrub.rb
index de4295e1..aff28009 100644
--- a/lib/loofah/html5/scrub.rb
+++ b/lib/loofah/html5/scrub.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'cgi'
 require 'crass'
 
diff --git a/lib/loofah/instance_methods.rb b/lib/loofah/instance_methods.rb
index c6366425..1a3249d0 100644
--- a/lib/loofah/instance_methods.rb
+++ b/lib/loofah/instance_methods.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Loofah
   #
   #  Mixes +scrub!+ into Document, DocumentFragment, Node and NodeSet.
diff --git a/lib/loofah/metahelpers.rb b/lib/loofah/metahelpers.rb
index 9949c77f..5eac24fe 100644
--- a/lib/loofah/metahelpers.rb
+++ b/lib/loofah/metahelpers.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Loofah
   module MetaHelpers # :nodoc:
     def self.add_downcased_set_members_to_all_set_constants mojule
diff --git a/lib/loofah/scrubber.rb b/lib/loofah/scrubber.rb
index 1137d58b..962850ca 100644
--- a/lib/loofah/scrubber.rb
+++ b/lib/loofah/scrubber.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Loofah
   #
   #  A RuntimeError raised when Loofah could not find an appropriate scrubber.
diff --git a/lib/loofah/scrubbers.rb b/lib/loofah/scrubbers.rb
index c6cba196..cce2fa02 100644
--- a/lib/loofah/scrubbers.rb
+++ b/lib/loofah/scrubbers.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Loofah
   #
   #  Loofah provides some built-in scrubbers for sanitizing with
diff --git a/lib/loofah/xml/document.rb b/lib/loofah/xml/document.rb
index bcc14389..9f00851f 100644
--- a/lib/loofah/xml/document.rb
+++ b/lib/loofah/xml/document.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Loofah
   module XML # :nodoc:
     #
diff --git a/lib/loofah/xml/document_fragment.rb b/lib/loofah/xml/document_fragment.rb
index 113b6bab..56b76731 100644
--- a/lib/loofah/xml/document_fragment.rb
+++ b/lib/loofah/xml/document_fragment.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Loofah
   module XML # :nodoc:
     #