diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 2d14d0f..5b95d17 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -30,7 +30,7 @@ jobs:
           script/install_linux_deps
 
       - name: Install MacOS packages
-        if: ${{ matrix.os == 'macos-11.0' }}
+        if: ${{ matrix.os == 'macos-latest' }}
         run: script/install_macos_deps
 
       - name: Set up Ruby ${{ matrix.ruby-version }}
diff --git a/.rubocop.yml b/.rubocop.yml
index 81748ee..2ae5ba2 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -2,3 +2,7 @@ inherit_gem:
   rubocop-standard:
     - config/default.yml
     - config/minitest.yml
+
+AllCops:
+  Exclude:
+    - 'ext/mathematical/mtex2MML/**/*'
diff --git a/Rakefile b/Rakefile
index 991cac5..53c10a0 100644
--- a/Rakefile
+++ b/Rakefile
@@ -29,7 +29,7 @@ Rake::Task[:test].prerequisites
 task build: [:clean]
 
 Rake::Task[:clean].enhance do
-  Dir.chdir(LASEM_DIR) { puts %x(git clean -fdx); }
+  Dir.chdir(LASEM_DIR) { puts %x(git clean -fdx) }
 end
 
 desc "Copy samples to gh-pages"
diff --git a/lib/mathematical/configuration.rb b/lib/mathematical/configuration.rb
index 53ceb8d..e4e1cac 100644
--- a/lib/mathematical/configuration.rb
+++ b/lib/mathematical/configuration.rb
@@ -19,6 +19,6 @@ def option_exists?(option)
           raise(TypeError, "delimiter type does not exist: #{option}")
         end
       end
-  end
+    end
   end
 end
diff --git a/lib/mathematical/corrections.rb b/lib/mathematical/corrections.rb
index 6becf14..4289dd1 100644
--- a/lib/mathematical/corrections.rb
+++ b/lib/mathematical/corrections.rb
@@ -11,7 +11,7 @@ def apply_corrections(maths)
     # inside itex equations. So "<" and ">" are significant.
     # To obtain a less-than or greater-than sign, you should use \lt or \gt, respectively.
     def adjust_lt_gt(maths)
-      maths.gsub(/</, '\lt').gsub(/>/, '\gt')
+      maths.gsub("<", '\lt').gsub(">", '\gt')
     end
   end
 end
diff --git a/script/install_linux_deps b/script/install_linux_deps
index d4a9565..3d5102c 100755
--- a/script/install_linux_deps
+++ b/script/install_linux_deps
@@ -3,7 +3,7 @@
 set -e
 export CC=gcc
 
-sudo apt-get -qq -y install bison flex libffi-dev libxml2-dev libgdk-pixbuf2.0-dev libcairo2-dev libpango1.0-dev
+sudo apt-get -qq -y install bison flex libffi-dev libxml2-dev libgdk-pixbuf2.0-dev libcairo2-dev libpango1.0-dev libwebp-dev
 
 # not necessary for tests, but is for any production/development environment
 if [[ ! -v CI ]]; then
diff --git a/test/mathematical/basic_test.rb b/test/mathematical/basic_test.rb
index f31a8ca..61dc2a9 100644
--- a/test/mathematical/basic_test.rb
+++ b/test/mathematical/basic_test.rb
@@ -3,8 +3,8 @@
 require "test_helper"
 require "nokogiri"
 
-module Mathematical
-  class BasicTest < MiniTest::Test
+class Mathematical
+  class BasicTest < Minitest::Test
     def test_it_has_a_version
       assert(Mathematical::VERSION)
     end
@@ -14,7 +14,7 @@ def test_multiple_calls
       render.render('$\pi$')
       output = render.render('$\pi$')[:data]
 
-      assert_equal(1, output.scan(/<svg/).size, "should only contain one svg")
+      assert_equal(1, output.scan("<svg").size, "should only contain one svg")
 
       # assert the SVG actually rendered
       doc = Nokogiri::HTML(output)
diff --git a/test/mathematical/corrections_test.rb b/test/mathematical/corrections_test.rb
index 115796a..6984582 100644
--- a/test/mathematical/corrections_test.rb
+++ b/test/mathematical/corrections_test.rb
@@ -2,8 +2,8 @@
 
 require "test_helper"
 
-module Mathematical
-  class CorrectionsTest < MiniTest::Test
+class Mathematical
+  class CorrectionsTest < Minitest::Test
     def setup
       @render = Mathematical.new
     end
diff --git a/test/mathematical/delimiters_test.rb b/test/mathematical/delimiters_test.rb
index c64e8cb..9776477 100644
--- a/test/mathematical/delimiters_test.rb
+++ b/test/mathematical/delimiters_test.rb
@@ -2,8 +2,8 @@
 
 require "test_helper"
 
-module Mathematical
-  class BasicTest < MiniTest::Test
+class Mathematical
+  class BasicTest < Minitest::Test
     def test_single_dollar
       render = Mathematical.new(delimiter: :DOLLAR, format: :mathml)
 
diff --git a/test/mathematical/fixtures_test.rb b/test/mathematical/fixtures_test.rb
index dc637a6..9a32e55 100644
--- a/test/mathematical/fixtures_test.rb
+++ b/test/mathematical/fixtures_test.rb
@@ -3,8 +3,8 @@
 require "test_helper"
 require "math-to-itex"
 
-module Mathematical
-  class FixturesTest < MiniTest::Test
+class Mathematical
+  class FixturesTest < Minitest::Test
     # the same SVGs sometimes get random id values, throwing off the tests
     def strip_id(blob)
       blob.gsub(/id="surface.+?"/, "")
@@ -17,7 +17,7 @@ def strip_id(blob)
         source = File.read(before)
 
         if ENV["MATHEMATICAL_GENERATE_SAMPLE"]
-          next unless /compliance/.match?(name)
+          next unless name.include?("compliance")
 
           write_sample(source)
         end
@@ -28,7 +28,7 @@ def strip_id(blob)
           %(<img class="#{type}-math" data-math-type="#{type}-math" src="#{svg_content[:data]}"/>)
         end.rstrip
 
-        expected_file = before.sub(/before/, "after").sub(/text/, "html")
+        expected_file = before.sub("before", "after").sub("text", "html")
 
         File.open(expected_file, "w") { |file| file.write(actual) } unless ENV["DEBUG_MATHEMATICAL"].nil?
 
diff --git a/test/mathematical/maliciousness_test.rb b/test/mathematical/maliciousness_test.rb
index 4bfd00e..a0cd9c1 100644
--- a/test/mathematical/maliciousness_test.rb
+++ b/test/mathematical/maliciousness_test.rb
@@ -2,8 +2,8 @@
 
 require "test_helper"
 
-module Mathematical
-  class MaliciousnessTest < MiniTest::Test
+class Mathematical
+  class MaliciousnessTest < Minitest::Test
     def test_it_does_not_error_on_unrecognized_commands
       render = Mathematical.new
       # In mtex2MML, we raise a ParseError, but Mathematical suppresses it and returns the string.
@@ -60,9 +60,8 @@ def test_it_does_not_blow_up_on_bad_options
       assert_equal('$a \ne b$', output[:data])
       assert_equal(output[:exception].class, Mathematical::MaxsizeError)
 
-      render = Mathematical.new({ maxsize: 2**80 })
       assert_raises(TypeError) do
-        render.render('$a \ne b$')
+        render = Mathematical.new({ maxsize: 2**80 })
       end
     end
 
@@ -123,11 +122,11 @@ def test_it_parses_all_possible_array_elements
       end
 
       assert_equal(3, output.length)
-      assert_equal(Hash, output.first.class)
-      assert_equal(Hash, output.last.class)
+      assert_instance_of(Hash, output.first)
+      assert_instance_of(Hash, output.last)
 
       assert_equal("$/this___istotallyfake$", output[1][:data])
-      assert_equal(Mathematical::ParseError, output[1][:exception].class)
+      assert_instance_of(Mathematical::ParseError, output[1][:exception])
       assert_match("Failed to parse mtex", output[1][:exception].message)
 
       # array errors also output to STDERR
@@ -145,7 +144,7 @@ def test_it_passes_a_legible_error_for_maxsize
       assert_equal(1, output.length)
 
       assert_equal('$a \ne b$', output[0][:data])
-      assert_equal(Mathematical::MaxsizeError, output[0][:exception].class)
+      assert_instance_of(Mathematical::MaxsizeError, output[0][:exception])
       assert_match("Size of latex string is greater than the maxsize", output[0][:exception].message)
 
       # array errors also output to STDERR
diff --git a/test/mathematical/mathjax_test.rb b/test/mathematical/mathjax_test.rb
index a4267d4..77a15f2 100644
--- a/test/mathematical/mathjax_test.rb
+++ b/test/mathematical/mathjax_test.rb
@@ -3,8 +3,8 @@
 require "test_helper"
 require "nokogiri"
 
-module Mathematical
-  class MathJaxTest < MiniTest::Test
+class Mathematical
+  class MathJaxTest < Minitest::Test
     render_svg = Mathematical.new
 
     SKIPPED = []
diff --git a/test/mathematical/mathml_test.rb b/test/mathematical/mathml_test.rb
index 96490a8..581b045 100644
--- a/test/mathematical/mathml_test.rb
+++ b/test/mathematical/mathml_test.rb
@@ -3,8 +3,8 @@
 # rubocop:disable Lint/ImplicitStringConcatenation
 require "test_helper"
 
-module Mathematical
-  class MathMLTest < MiniTest::Test
+class Mathematical
+  class MathMLTest < Minitest::Test
     def test_it_returns_mathml
       string = ""'
     $$
diff --git a/test/mathematical/multiples_test.rb b/test/mathematical/multiples_test.rb
index e875bdf..962e1eb 100644
--- a/test/mathematical/multiples_test.rb
+++ b/test/mathematical/multiples_test.rb
@@ -2,8 +2,8 @@
 
 require "test_helper"
 
-module Mathematical
-  class MultiplesTest < MiniTest::Test
+class Mathematical
+  class MultiplesTest < Minitest::Test
     def setup
       @render = Mathematical.new(base64: true)
     end
@@ -20,7 +20,7 @@ def test_it_works_for_a_single_numbered_equation
       output = @render.render([string])
       svg = output.first[:data]
 
-      assert_equal(1, svg.scan(/svg\+xml;/).size, "should only contain one svg")
+      assert_equal(1, svg.scan("svg+xml;").size, "should only contain one svg")
       assert_match("PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My", svg)
     end
 
@@ -46,6 +46,7 @@ def test_it_works_for_many_equations
     end
 
     def test_it_properly_accounts_for_equations
+      skip("png tests are currently unsupported")
       inputs = []
       (1..2).each do |i|
         string = """
diff --git a/test/mathematical/performance_test.rb b/test/mathematical/performance_test.rb
index 35d0235..b559042 100644
--- a/test/mathematical/performance_test.rb
+++ b/test/mathematical/performance_test.rb
@@ -3,8 +3,8 @@
 require "test_helper"
 require "benchmark"
 
-module Mathematical
-  class BasicTest < MiniTest::Test
+class Mathematical
+  class BasicTest < Minitest::Test
     def test_it_handles_big_files
       big_file = File.read("test/mathematical/fixtures/performance/big_file.text")
       speed = Benchmark.realtime do
diff --git a/test/mathematical/png_test.rb b/test/mathematical/png_test.rb
index 545d22d..ea8e4ec 100644
--- a/test/mathematical/png_test.rb
+++ b/test/mathematical/png_test.rb
@@ -4,13 +4,14 @@
 
 require "test_helper"
 
-module Mathematical
-  class PNGTest < MiniTest::Test
+class Mathematical
+  class PNGTest < Minitest::Test
     def before
       File.delete("#{fixtures_dir}/png/pmatrix.png") if File.exist?("#{fixtures_dir}/png/pmatrix.png")
     end
 
     def test_it_creates_a_png
+      skip("png tests are currently unsupported")
       string = ""'
     $$
 \begin{pmatrix}