diff --git a/core/unboundmethod/equal_value_spec.rb b/core/unboundmethod/equal_value_spec.rb
index 4d4fc66504..b2d78c50af 100644
--- a/core/unboundmethod/equal_value_spec.rb
+++ b/core/unboundmethod/equal_value_spec.rb
@@ -81,7 +81,7 @@
     (@child1 == @parent).should == true
   end
 
-  it "returns false if same method but extracted from two different subclasses" do
+  it "returns true if same method but extracted from two different subclasses" do
     (@child2 == @child1).should == true
     (@child1 == @child2).should == true
   end
diff --git a/library/cgi/escapeURIComponent_spec.rb b/library/cgi/escapeURIComponent_spec.rb
index 02921ab8bf..1cea2b786a 100644
--- a/library/cgi/escapeURIComponent_spec.rb
+++ b/library/cgi/escapeURIComponent_spec.rb
@@ -6,48 +6,67 @@
 end
 
 describe "CGI.escapeURIComponent" do
-  it "escapes whitespace" do
-    string = "&<>\" \xE3\x82\x86\xE3\x82\x93\xE3\x82\x86\xE3\x82\x93"
-    CGI.escapeURIComponent(string).should == '%26%3C%3E%22%20%E3%82%86%E3%82%93%E3%82%86%E3%82%93'
+  it "percent-encodes characters reserved according to RFC 3986" do
+    # https://www.rfc-editor.org/rfc/rfc3986#section-2.2
+    string = ":/?#[]@!$&'()*+,;="
+    CGI.escapeURIComponent(string).should == "%3A%2F%3F%23%5B%5D%40%21%24%26%27%28%29%2A%2B%2C%3B%3D"
   end
 
-  it "does not escape with unreserved characters" do
+  it "does not percent-encode unreserved characters according to RFC 3986" do
     string = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~"
     CGI.escapeURIComponent(string).should == "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~"
   end
 
-  it "supports String with invalid encoding" do
-    string = "\xC0\<\<".dup.force_encoding("UTF-8")
-    CGI.escapeURIComponent(string).should == "%C0%3C%3C"
+  it "encodes % character as %25" do
+    CGI.escapeURIComponent("%").should == "%25"
   end
 
-  it "processes String bytes one by one, not characters" do
-    CGI.escapeURIComponent("β").should == "%CE%B2" # "β" bytes representation is CE B2
+  # Compare to .escape which uses "+".
+  it "percent-encodes single whitespace" do
+    CGI.escapeURIComponent(" ").should == "%20"
   end
 
-  it "raises a TypeError with nil" do
-    -> {
-      CGI.escapeURIComponent(nil)
-    }.should raise_error(TypeError, 'no implicit conversion of nil into String')
+  it "percent-encodes all non-reserved and non-unreserved ASCII characters" do
+    special_set = ":/?#[]@!$&'()*+,;=ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-._~"
+    all_other = (0x00..0x7F).filter_map { |i| i.chr unless special_set.include?(i.chr) }.join
+    encoded = CGI.escapeURIComponent(all_other)
+    encoded.should.match?(/\A(?:%[0-9A-F]{2}){#{all_other.length}}\z/)
   end
 
-  it "encodes empty string" do
-    CGI.escapeURIComponent("").should == ""
+  it "percent-encodes non-ASCII bytes" do
+    bytes = (0x80..0xFF).map(&:chr).join
+    encoded = CGI.escapeURIComponent(bytes)
+    encoded.should.match?(/\A(?:%[0-9A-F]{2}){#{bytes.length}}\z/)
   end
 
-  it "encodes single whitespace" do
-    CGI.escapeURIComponent(" ").should == "%20"
+  it "processes multi-byte characters as separate bytes, percent-encoding each one" do
+    CGI.escapeURIComponent("β").should == "%CE%B2" # "β" bytes representation is CE B2
   end
 
-  it "encodes double whitespace" do
-    CGI.escapeURIComponent("  ").should == "%20%20"
+  it "produces a copy of an empty string" do
+    string = "".encode(Encoding::BINARY)
+    encoded = CGI.escapeURIComponent(string)
+    encoded.should == ""
+    encoded.encoding.should == Encoding::BINARY
+    string.should_not.equal?(encoded)
   end
 
-  it "preserves encoding" do
+  it "preserves string's encoding" do
     string = "whatever".encode("ASCII-8BIT")
     CGI.escapeURIComponent(string).encoding.should == Encoding::ASCII_8BIT
   end
 
+  it "processes even strings with invalid encoding, percent-encoding octets as-is" do
+    string = "\xC0<<".dup.force_encoding("UTF-8")
+    CGI.escapeURIComponent(string).should == "%C0%3C%3C"
+  end
+
+  it "raises a TypeError with nil" do
+    -> {
+      CGI.escapeURIComponent(nil)
+    }.should raise_error(TypeError, "no implicit conversion of nil into String")
+  end
+
   it "uses implicit type conversion to String" do
     object = Object.new
     def object.to_str
diff --git a/library/cgi/unescapeURIComponent_spec.rb b/library/cgi/unescapeURIComponent_spec.rb
new file mode 100644
index 0000000000..f80eb1626b
--- /dev/null
+++ b/library/cgi/unescapeURIComponent_spec.rb
@@ -0,0 +1,128 @@
+require_relative '../../spec_helper'
+
+ruby_version_is ""..."4.0" do
+  require 'cgi'
+end
+ruby_version_is "4.0" do
+  require 'cgi/escape'
+end
+
+describe "CGI.unescapeURIComponent" do
+  it "decodes any percent-encoded octets to their corresponding bytes according to RFC 3986" do
+    string = (0x00..0xff).map { |i| "%%%02x" % i }.join
+    expected = (0x00..0xff).map { |i| i.chr }.join.force_encoding(Encoding::UTF_8)
+    CGI.unescapeURIComponent(string).should == expected
+  end
+
+  it "disregards case of characters in a percent-encoding triplet" do
+    CGI.unescapeURIComponent("%CE%B2abc").should == "βabc"
+    CGI.unescapeURIComponent("%ce%b2ABC").should == "βABC"
+  end
+
+  it "leaves any non-percent-encoded characters as-is" do
+    string = "ABCDEFGHIJKLMNOPQRSTUVWXYZ:/?#[]@!$&'()*+,;=\t\x0D\xFFβᛉ▒90%"
+    decoded = CGI.unescapeURIComponent(string)
+    decoded.should == string
+    string.should_not.equal?(decoded)
+  end
+
+  it "leaves sequences which can't be a percent-encoded octet as-is" do
+    string = "%AZ%B"
+    decoded = CGI.unescapeURIComponent(string)
+    decoded.should == string
+    string.should_not.equal?(decoded)
+  end
+
+  it "creates a String with the specified target Encoding" do
+    string = CGI.unescapeURIComponent("%D2%3C%3CABC", Encoding::ISO_8859_1)
+    string.encoding.should == Encoding::ISO_8859_1
+    string.should == "Ò<<ABC".encode("ISO-8859-1")
+  end
+
+  it "accepts a string name of an Encoding" do
+    CGI.unescapeURIComponent("%D2%3C%3CABC", "ISO-8859-1").should == "Ò<<ABC".encode("ISO-8859-1")
+  end
+
+  it "raises ArgumentError if specified encoding is unknown" do
+    -> { CGI.unescapeURIComponent("ABC", "ISO-JOKE-1") }.should raise_error(ArgumentError, "unknown encoding name - ISO-JOKE-1")
+  end
+
+  ruby_version_is ""..."4.0" do
+    it "uses CGI.accept_charset as the default target encoding" do
+      original_charset = CGI.accept_charset
+      CGI.accept_charset = "ISO-8859-1"
+      decoded = CGI.unescapeURIComponent("%D2%3C%3CABC")
+      decoded.should == "Ò<<ABC".encode("ISO-8859-1")
+      decoded.encoding.should == Encoding::ISO_8859_1
+    ensure
+      CGI.accept_charset = original_charset
+    end
+
+    it "has CGI.accept_charset as UTF-8 by default" do
+      decoded = CGI.unescapeURIComponent("%CE%B2ABC")
+      decoded.should == "βABC"
+      decoded.encoding.should == Encoding::UTF_8
+    end
+  end
+
+  ruby_version_is "4.0" do
+    # "cgi/escape" does not have methods to access @@accept_charset.
+    # Full "cgi" gem provides them, allowing to possibly change it.
+    it "uses CGI's @@accept_charset as the default target encoding" do
+      original_charset = CGI.class_variable_get(:@@accept_charset)
+      CGI.class_variable_set(:@@accept_charset, "ISO-8859-1")
+      decoded = CGI.unescapeURIComponent("%D2%3C%3CABC")
+      decoded.should == "Ò<<ABC".encode("ISO-8859-1")
+      decoded.encoding.should == Encoding::ISO_8859_1
+    ensure
+      CGI.class_variable_set(:@@accept_charset, original_charset)
+    end
+
+    it "has CGI's @@accept_charset as UTF-8 by default" do
+      decoded = CGI.unescapeURIComponent("%CE%B2ABC")
+      decoded.should == "βABC"
+      decoded.encoding.should == Encoding::UTF_8
+    end
+  end
+
+  context "when source string specifies octets invalid in target encoding" do
+    it "uses source string's encoding" do
+      string = "%A2%A6%A3".encode(Encoding::SHIFT_JIS)
+      decoded = CGI.unescapeURIComponent(string, Encoding::US_ASCII)
+      decoded.encoding.should == Encoding::SHIFT_JIS
+      decoded.should == "｢ｦ｣".encode(Encoding::SHIFT_JIS)
+      decoded.valid_encoding?.should be_true
+    end
+
+    it "uses source string's encoding even if it's also invalid" do
+      string = "%FF".encode(Encoding::US_ASCII)
+      decoded = CGI.unescapeURIComponent(string, Encoding::SHIFT_JIS)
+      decoded.encoding.should == Encoding::US_ASCII
+      decoded.should == "\xFF".dup.force_encoding(Encoding::US_ASCII)
+      decoded.valid_encoding?.should be_false
+    end
+  end
+
+  it "decodes an empty string as an empty string with target encoding" do
+    string = "".encode(Encoding::BINARY)
+    decoded = CGI.unescapeURIComponent(string, "UTF-8")
+    decoded.should == ""
+    decoded.encoding.should == Encoding::UTF_8
+    string.should_not.equal?(decoded)
+  end
+
+  it "raises a TypeError with nil" do
+    -> {
+      CGI.unescapeURIComponent(nil)
+    }.should raise_error(TypeError, "no implicit conversion of nil into String")
+  end
+
+  it "uses implicit type conversion to String" do
+    object = Object.new
+    def object.to_str
+      "a%20b"
+    end
+
+    CGI.unescapeURIComponent(object).should == "a b"
+  end
+end