diff --git a/.rubocop.yml b/.rubocop.yml
index f005b3a..a30ab8e 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -1,3 +1,6 @@
+require:
+  - rubocop-rake
+
 AllCops:
   TargetRubyVersion: 2.7  # lowest supported version
   NewCops: enable
@@ -53,6 +56,10 @@ Style/Not:
 Style/NumericPredicate:
   Enabled: false
 
+Style/OpenStructUse:
+  Exclude:
+    - 'test/**/*'
+
 Style/RedundantFreeze:
   Enabled: false
 
diff --git a/Gemfile b/Gemfile
index 1d4e592..1ca5eff 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,3 +5,12 @@
 source 'https://rubygems.org'
 
 gemspec
+
+gem 'benchmark-ips'
+gem 'bump'
+gem 'maxitest'
+gem 'memory_profiler'
+gem 'rake'
+gem 'rubocop'
+gem 'rubocop-rake'
+gem 'webrick'
diff --git a/Gemfile.lock b/Gemfile.lock
index c173c50..b6f2222 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -9,37 +9,45 @@ GEM
   specs:
     addressable (2.8.6)
       public_suffix (>= 2.0.2, < 6.0)
-    ast (2.4.1)
-    benchmark-ips (2.8.4)
+    ast (2.4.2)
+    benchmark-ips (2.13.0)
     bump (0.10.0)
-    maxitest (3.6.0)
-      minitest (>= 5.0.0, < 5.14.0)
-    memory_profiler (1.0.0)
-    minitest (5.12.0)
-    parallel (1.20.1)
-    parser (3.0.0.0)
+    json (2.7.2)
+    json (2.7.2-java)
+    language_server-protocol (3.17.0.3)
+    maxitest (4.5.0)
+      minitest (>= 5.0.0, < 5.19.0)
+    memory_profiler (1.0.1)
+    minitest (5.18.1)
+    parallel (1.24.0)
+    parser (3.3.0.5)
       ast (~> 2.4.1)
+      racc
     public_suffix (5.0.5)
-    rainbow (3.0.0)
-    rake (13.0.3)
-    regexp_parser (2.0.3)
-    rexml (3.2.4)
-    rubocop (1.8.0)
+    racc (1.7.3)
+    racc (1.7.3-java)
+    rainbow (3.1.1)
+    rake (13.2.1)
+    regexp_parser (2.9.0)
+    rexml (3.2.6)
+    rubocop (1.62.1)
+      json (~> 2.3)
+      language_server-protocol (>= 3.17.0)
       parallel (~> 1.10)
-      parser (>= 3.0.0.0)
+      parser (>= 3.3.0.2)
       rainbow (>= 2.2.2, < 4.0)
       regexp_parser (>= 1.8, < 3.0)
-      rexml
-      rubocop-ast (>= 1.2.0, < 2.0)
+      rexml (>= 3.2.5, < 4.0)
+      rubocop-ast (>= 1.31.1, < 2.0)
       ruby-progressbar (~> 1.7)
-      unicode-display_width (>= 1.4.0, < 3.0)
-    rubocop-ast (1.4.0)
-      parser (>= 2.7.1.5)
-    rubocop-rake (0.5.1)
-      rubocop
-    ruby-progressbar (1.11.0)
-    unicode-display_width (2.0.0)
-    webrick (1.7.0)
+      unicode-display_width (>= 2.4.0, < 3.0)
+    rubocop-ast (1.31.2)
+      parser (>= 3.3.0.4)
+    rubocop-rake (0.6.0)
+      rubocop (~> 1.0)
+    ruby-progressbar (1.13.0)
+    unicode-display_width (2.5.0)
+    webrick (1.8.1)
 
 PLATFORMS
   java
@@ -52,9 +60,9 @@ DEPENDENCIES
   maxitest
   memory_profiler
   rake
-  rubocop (~> 1.8)
+  rubocop
   rubocop-rake
   webrick
 
 BUNDLED WITH
-   2.1.4
+   2.4.12
diff --git a/css_parser.gemspec b/css_parser.gemspec
index f3a12d8..242c11c 100644
--- a/css_parser.gemspec
+++ b/css_parser.gemspec
@@ -19,13 +19,4 @@ Gem::Specification.new name, CssParser::VERSION do |s|
   s.metadata['rubygems_mfa_required'] = 'true'
 
   s.add_runtime_dependency 'addressable'
-
-  s.add_development_dependency 'benchmark-ips'
-  s.add_development_dependency 'bump'
-  s.add_development_dependency 'maxitest'
-  s.add_development_dependency 'memory_profiler'
-  s.add_development_dependency 'rake'
-  s.add_development_dependency 'rubocop', '~> 1.8'
-  s.add_development_dependency 'rubocop-rake'
-  s.add_development_dependency 'webrick'
 end
diff --git a/lib/css_parser.rb b/lib/css_parser.rb
index a852fe6..b50f81b 100644
--- a/lib/css_parser.rb
+++ b/lib/css_parser.rb
@@ -111,7 +111,7 @@ def self.merge(*rule_sets)
   #++
   def self.calculate_specificity(selector)
     a = 0
-    b = selector.scan(/\#/).length
+    b = selector.scan('#').length
     c = selector.scan(NON_ID_ATTRIBUTES_AND_PSEUDO_CLASSES_RX_NC).length
     d = selector.scan(ELEMENTS_AND_PSEUDO_ELEMENTS_RX_NC).length
 
diff --git a/lib/css_parser/parser.rb b/lib/css_parser/parser.rb
index b5e203a..357b79e 100644
--- a/lib/css_parser/parser.rb
+++ b/lib/css_parser/parser.rb
@@ -133,7 +133,7 @@ def add_block!(block, options = {})
         block.scan(RE_AT_IMPORT_RULE).each do |import_rule|
           media_types = []
           if (media_string = import_rule[-1])
-            media_string.split(/,/).each do |t|
+            media_string.split(',').each do |t|
               media_types << CssParser.sanitize_media_query(t) unless t.empty?
             end
           else
diff --git a/lib/css_parser/regexps.rb b/lib/css_parser/regexps.rb
index 0529173..cf83b2d 100644
--- a/lib/css_parser/regexps.rb
+++ b/lib/css_parser/regexps.rb
@@ -11,7 +11,7 @@ def self.regex_possible_values(*values)
   RE_NON_ASCII = Regexp.new('([\x00-\xFF])', Regexp::IGNORECASE | Regexp::NOENCODING) # [^\0-\177]
   RE_UNICODE = Regexp.new('(\\\\[0-9a-f]{1,6}(\r\n|[ \n\r\t\f])*)', Regexp::IGNORECASE | Regexp::EXTENDED | Regexp::MULTILINE | Regexp::NOENCODING)
   RE_ESCAPE = Regexp.union(RE_UNICODE, '|(\\\\[^\n\r\f0-9a-f])')
-  RE_IDENT = Regexp.new("[\-]?([_a-z]|#{RE_NON_ASCII}|#{RE_ESCAPE})([_a-z0-9\-]|#{RE_NON_ASCII}|#{RE_ESCAPE})*", Regexp::IGNORECASE | Regexp::NOENCODING)
+  RE_IDENT = Regexp.new("[-]?([_a-z]|#{RE_NON_ASCII}|#{RE_ESCAPE})([_a-z0-9-]|#{RE_NON_ASCII}|#{RE_ESCAPE})*", Regexp::IGNORECASE | Regexp::NOENCODING)
 
   # General strings
   RE_STRING1 = /("(.[^\n\r\f"]*|\\#{RE_NL}|#{RE_ESCAPE})*")/.freeze
diff --git a/lib/css_parser/rule_set.rb b/lib/css_parser/rule_set.rb
index 3b445cd..c3904c1 100644
--- a/lib/css_parser/rule_set.rb
+++ b/lib/css_parser/rule_set.rb
@@ -58,7 +58,7 @@ def ==(other)
 
       extend Forwardable
 
-      def_delegators :declarations, :each
+      def_delegators :declarations, :each, :each_value
 
       def initialize(declarations = {})
         self.declarations = {}
@@ -142,7 +142,7 @@ def replace_declaration!(property, replacements, preserve_importance: false)
 
         if preserve_importance
           importance = get_value(property).important
-          replacement_declarations.each { |_key, value| value.important = importance }
+          replacement_declarations.each_value { |value| value.important = importance }
         end
 
         replacement_keys = declarations.keys
@@ -308,7 +308,7 @@ def expand_background_shorthand! # :nodoc:
 
       replacement =
         if value.match(CssParser::RE_INHERIT)
-          BACKGROUND_PROPERTIES.map { |key| [key, 'inherit'] }.to_h
+          BACKGROUND_PROPERTIES.to_h { |key| [key, 'inherit'] }
         else
           {
             'background-image' => value.slice!(CssParser::RE_IMAGE),
@@ -448,7 +448,7 @@ def expand_list_style_shorthand! # :nodoc:
 
       replacement =
         if value =~ CssParser::RE_INHERIT
-          LIST_STYLE_PROPERTIES.map { |key| [key, 'inherit'] }.to_h
+          LIST_STYLE_PROPERTIES.to_h { |key| [key, 'inherit'] }
         else
           {
             'list-style-type' => value.slice!(CssParser::RE_LIST_STYLE_TYPE),
diff --git a/test/rule_set/test_declarations.rb b/test/rule_set/test_declarations.rb
index d2d2a4d..8158787 100644
--- a/test/rule_set/test_declarations.rb
+++ b/test/rule_set/test_declarations.rb
@@ -167,7 +167,7 @@ class RuleSetDeclarationsTest < Minitest::Test
       declarations = CssParser::RuleSet::Declarations.new({foo: 'foo value'})
       assert_equal 1, declarations.size
 
-      declarations.remove_declaration!('fOo'.to_sym)
+      declarations.remove_declaration!(:fOo)
 
       assert_equal 0, declarations.size
     end
diff --git a/test/test_css_parser_media_types.rb b/test/test_css_parser_media_types.rb
index 0784ae2..054d1b1 100644
--- a/test/test_css_parser_media_types.rb
+++ b/test/test_css_parser_media_types.rb
@@ -43,7 +43,7 @@ def test_finding_by_media_type
 
     assert_equal 'font-size: 10pt; line-height: 1.2;', @cp.find_by_selector('body', :print).join(' ')
     assert_equal 'font-size: 13px; line-height: 1.2; color: blue;', @cp.find_by_selector('body', :screen).join(' ')
-    assert_equal 'color: blue;', @cp.find_by_selector('body', 'print and resolution > 90dpi'.to_sym).join(' ')
+    assert_equal 'color: blue;', @cp.find_by_selector('body', :'print and resolution > 90dpi').join(' ')
   end
 
   def test_with_parenthesized_media_features
diff --git a/test/test_css_parser_misc.rb b/test/test_css_parser_misc.rb
index 3756c8c..758355a 100644
--- a/test/test_css_parser_misc.rb
+++ b/test/test_css_parser_misc.rb
@@ -79,7 +79,7 @@ def test_parsing_blocks
       \
           foo(int x) {\
               this.x = 'test';\
-              this.x = \"test\";\
+              this.x = "test";\
           }\
       \
       }"] { color: red }
@@ -127,8 +127,7 @@ def test_multiline_declarations
     @cp.each_selector do |selector, declarations, _spec|
       assert_equal '@font-face', selector
       assert_equal "font-family: 'some_font'; " \
-                   "src: url(https://example.com/font.woff2) format('woff2')," \
-                        "url(https://example.com/font.woff) format('woff'); " \
+                   "src: url(https://example.com/font.woff2) format('woff2'),url(https://example.com/font.woff) format('woff'); " \
                    "font-style: normal;", declarations
     end
   end
diff --git a/test/test_rule_set.rb b/test/test_rule_set.rb
index 90eae0d..e8bf458 100644
--- a/test/test_rule_set.rb
+++ b/test/test_rule_set.rb
@@ -73,7 +73,7 @@ def test_each_declaration_respects_order
 
   def test_each_declaration_containing_semicolons
     rs = RuleSet.new(nil, "background-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABwAAAAiCAMAAAB7);" \
-      "background-repeat: no-repeat")
+                          "background-repeat: no-repeat")
     assert_equal('url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABwAAAAiCAMAAAB7);', rs['background-image'])
     assert_equal('no-repeat;', rs['background-repeat'])
   end
diff --git a/test/test_rule_set_creating_shorthand.rb b/test/test_rule_set_creating_shorthand.rb
index d15aaa1..0fce89c 100644
--- a/test/test_rule_set_creating_shorthand.rb
+++ b/test/test_rule_set_creating_shorthand.rb
@@ -219,7 +219,7 @@ def test_a_single_property_is_not_shorted
 protected
 
   def assert_properties_are_deleted(ruleset, properties)
-    properties.each do |property, _value|
+    properties.each_key do |property|
       assert_equal '', ruleset[property]
     end
   end
diff --git a/test/test_rule_set_expanding_shorthand.rb b/test/test_rule_set_expanding_shorthand.rb
index 792285e..d7b3887 100644
--- a/test/test_rule_set_expanding_shorthand.rb
+++ b/test/test_rule_set_expanding_shorthand.rb
@@ -247,7 +247,7 @@ def test_getting_list_style_properties_from_shorthand
       'list-style-position' => 'inside'
     }
 
-    shorthand = "list-style: katakana inside url(\'chess.png\');"
+    shorthand = "list-style: katakana inside url('chess.png');"
     declarations = expand_declarations(shorthand)
     assert_equal expected, declarations
   end