Add assert_equivalent_html

Gemfile

@@ -7,11 +7,11 @@ gemspec
 
 gem "rspec-expectations"
 gem "minitest"
+gem "nokogiri"
 
 if RUBY_ENGINE == "ruby"
 	group :development do
 		gem "rubocop"
-		gem "minitest"
 		gem "sus"
 		gem "covered"
 	end

lib/quickdraw.rb

@@ -18,6 +18,7 @@ module Quickdraw
 	autoload :Timer, "quickdraw/timer"
 	autoload :Worker, "quickdraw/worker"
 	autoload :MinitestAdapter, "quickdraw/minitest_adapter"
+	autoload :HTMLPrettifier, "quickdraw/html_prettifier"
 
 	Null = Object.new.freeze
 	Error = Module.new

lib/quickdraw/assertions.rb

@@ -36,10 +36,33 @@ def assert_equal_html(actual, expected)
 		assert(actual == expected) do
 			diff = DIFFER.diff_html(actual, expected)
 
+			if diff.end_with?("No syntactic changes.")
+				diff = DIFFER.diff_strings(actual, expected)
+			end
+
 			"Expected HTML strings to be equal (compared with `actual == expected`):\n\n#{diff}"
 		end
 	end
 
+	def assert_equivalent_html(actual, expected)
+		if actual == expected
+			success!
+		else
+			pretty_actual = Quickdraw::HTMLPrettifier.prettify(actual)
+			pretty_expected = Quickdraw::HTMLPrettifier.prettify(expected)
+
+			assert(pretty_actual == pretty_expected) do
+				diff = DIFFER.diff_html(pretty_actual, pretty_expected)
+
+				if diff.end_with?("No syntactic changes.")
+					diff = DIFFER.diff_strings(pretty_actual, pretty_expected)
+				end
+
+				"Expected HTML to be equivalent (ignoring whitespace):\n\n#{diff}"
+			end
+		end
+	end
+
 	def assert_equal_ruby(actual, expected)
 		unless String === actual && String === expected
 			raise ArgumentError.new("expected both actual and expected to be strings")

lib/quickdraw/html_prettifier.rb

@@ -0,0 +1,74 @@
+# frozen_string_literal: true
+
+require "nokogiri"
+
+module Quickdraw::HTMLPrettifier
+	extend self
+
+	VOID_ELEMENTS = Set[
+		"area",
+		"base",
+		"br",
+		"col",
+		"embed",
+		"hr",
+		"img",
+		"input",
+		"link",
+		"meta",
+		"source",
+		"track",
+	].freeze
+
+	def prettify(html)
+		node = case html
+		when Nokogiri::HTML5::DocumentFragment
+			html
+		when String
+			Nokogiri::HTML5.fragment(html)
+		else
+			raise ArgumentError.new("Expected a Nokogiri::HTML5::DocumentFragment or a String, but got a #{html.class}")
+		end
+
+		prettify_node(node)
+	end
+
+	def prettify_node(node, depth = 0)
+		indent = "  " * depth
+
+		case node
+		when Nokogiri::HTML5::DocumentFragment
+			map_children(node, depth)
+		when Nokogiri::XML::Element
+			attributes = node.attribute_nodes.map { |attr| prettify_node(attr) }.join
+			opening_tag = "#{indent}<#{node.name}#{attributes}>"
+
+			if VOID_ELEMENTS.include?(node.name)
+				opening_tag
+			else
+				[
+					opening_tag,
+					map_children(node, depth + 1),
+					"#{indent}</#{node.name}>",
+				].freeze.compact.join("\n")
+			end
+		when Nokogiri::XML::Text
+			text = node.text.strip
+			"#{indent}#{text}" if text.length > 0
+		when Nokogiri::XML::Attr
+			if node.value == ""
+				" #{node.name}"
+			else
+				%( #{node.name}="#{node.value}")
+			end
+		else
+			raise ArgumentError.new("Unexpected node type: #{node.class}")
+		end
+	end
+
+	def map_children(node, depth)
+		if node.children.any?
+			node.children.map { |child| prettify_node(child, depth) }.compact.join("\n")
+		end
+	end
+end