Hash#slice! was not implemented as I expected.

.travis.yml

@@ -4,6 +4,7 @@ cache: bundler
 
 matrix:
   include:
+    - rvm: 2.4
     - rvm: 2.5
     - rvm: 2.6
     - rvm: 2.6

lib/trenni/sanitize/filter.rb

@@ -66,6 +66,10 @@ def accept!(mode = ALL)
 				def [] key
 					self.tag&.attributes[key]
 				end
+
+				def limit_attributes(keys)
+					self.tag&.attributes&.select!{|key, value| keys.include?(key)}
+				end
 			end
 
 			def initialize(output, entities)

lib/trenni/sanitize/fragment.rb

@@ -20,10 +20,12 @@
 
 require_relative 'filter'
 
+require 'set'
+
 module Trenni
 	module Sanitize
 		class Fragment < Filter
-			STANDARD_ATTRIBUTES = ['class', 'style'].freeze
+			STANDARD_ATTRIBUTES = Set.new(['class', 'style']).freeze
 
 			ALLOWED_TAGS = {
 				'div' => STANDARD_ATTRIBUTES,
@@ -49,7 +51,7 @@ class Fragment < Filter
 
 			def filter(node)
 				if attributes = ALLOWED_TAGS[node.name]
-					node.tag.attributes.slice!(*attributes)
+					node.limit_attributes(attributes)
 
 					node.accept!
 				else

trenni-sanitize.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |spec|
 	spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
 	spec.require_paths = ["lib"]
 
-	spec.required_ruby_version = '~> 2.5'
+	spec.required_ruby_version = '~> 2.4'
 
 	spec.add_dependency "trenni", '~> 3.5'