diff --git a/lib/rexml/attribute.rb b/lib/rexml/attribute.rb
index c5673249..b31beb74 100644
--- a/lib/rexml/attribute.rb
+++ b/lib/rexml/attribute.rb
@@ -180,7 +180,7 @@ def element=( element )
     #
     # This method is usually not called directly.
     def remove
-      @element.attributes.delete self.name unless @element.nil?
+      @element.attributes.delete self unless @element.nil?
     end
 
     # Writes this attribute (EG, puts 'key="value"' to the output)
@@ -205,6 +205,15 @@ def xpath
     def document
       @element&.document
     end
+
+    # Returns true if this attribute is a namespace declaration, false otherwise.
+    #   "foo" => false
+    #   "xmlns" => true
+    #   "xmlns:foo" => true
+    #   "foo:xmlns" => false
+    def namespace_declaration?
+      prefix == 'xmlns' || (prefix.empty? && name == 'xmlns')
+    end
   end
 end
 #vim:ts=2 sw=2 noexpandtab:
diff --git a/lib/rexml/doctype.rb b/lib/rexml/doctype.rb
index d4f0bbe4..f58a3929 100644
--- a/lib/rexml/doctype.rb
+++ b/lib/rexml/doctype.rb
@@ -113,23 +113,27 @@ def node_type
     end
 
     def attributes_of element
-      rv = []
-      each do |child|
-        child.each do |key,val|
-          rv << Attribute.new(key,val)
-        end if child.kind_of? AttlistDecl and child.element_name == element
+      attribute_declarations_of(element).map do |key, value|
+        Attribute.new(key, value)
       end
-      rv
     end
 
     def attribute_of element, attribute
-      att_decl = find do |child|
-        child.kind_of? AttlistDecl and
-        child.element_name == element and
-        child.include? attribute
+      attribute_declarations_of(element)[attribute]
+    end
+
+    def attribute_declarations_of(name)
+      raw_attributes = {}
+      decls = select do |child|
+        child.kind_of?(AttlistDecl) && child.element_name == name
+      end
+      decls.each do |child|
+        child.each do |key, val|
+          # First declaration wins
+          raw_attributes[key] = val unless raw_attributes.key? key
+        end
       end
-      return nil unless att_decl
-      att_decl[attribute]
+      raw_attributes
     end
 
     def clone
diff --git a/lib/rexml/element.rb b/lib/rexml/element.rb
index abdb28a7..ef1bf976 100644
--- a/lib/rexml/element.rb
+++ b/lib/rexml/element.rb
@@ -2240,15 +2240,10 @@ def length
     #   [REXML::Attribute, bar:att='2']
     #   [REXML::Attribute, att='&lt;']
     #
-    def each_attribute # :yields: attribute
-      return to_enum(__method__) unless block_given?
-      each_value do |val|
-        if val.kind_of? Attribute
-          yield val
-        else
-          val.each_value { |atr| yield atr }
-        end
-      end
+    # This method doesn't iterate attributes in the DTD. This may be a bug.
+    #
+    def each_attribute(&block) # :yields: attribute
+      each_own_attribute(&block)
     end
 
     # :call-seq:
@@ -2273,6 +2268,8 @@ def each_attribute # :yields: attribute
     #   ["bar:att", "2"]
     #   ["att", "<"]
     #
+    # This method doesn't iterate attributes in the DTD. This may be a bug.
+    #
     def each
       return to_enum(__method__) unless block_given?
       each_attribute do |attr|
@@ -2300,36 +2297,7 @@ def each
     #   attrs.get_attribute('nosuch')        # => nil
     #
     def get_attribute( name )
-      attr = fetch( name, nil )
-      if attr.nil?
-        return nil if name.nil?
-        # Look for prefix
-        name =~ Namespace::NAMESPLIT
-        prefix, n = $1, $2
-        if prefix
-          attr = fetch( n, nil )
-          # check prefix
-          if attr == nil
-          elsif attr.kind_of? Attribute
-            return attr if prefix == attr.prefix
-          else
-            attr = attr[ prefix ]
-            return attr
-          end
-        end
-        doctype = @element.document&.doctype
-        if doctype
-          expn = @element.expanded_name
-          expn = doctype.name if expn.size == 0
-          attr_val = doctype.attribute_of(expn, name)
-          return Attribute.new( name, attr_val ) if attr_val
-        end
-        return nil
-      end
-      if attr.kind_of? Hash
-        attr = attr[ @element.prefix ]
-      end
-      attr
+      fetch(name, nil) || attlist_attributes&.[](name)
     end
 
     # :call-seq:
@@ -2357,8 +2325,7 @@ def get_attribute( name )
     #
     def []=( name, value )
       if value.nil?             # Delete the named attribute
-        attr = get_attribute(name)
-        delete attr
+        delete name
         return
       end
 
@@ -2372,17 +2339,7 @@ def []=( name, value )
         value = Attribute.new(name, value)
       end
       value.element = @element
-      old_attr = fetch(value.name, nil)
-      if old_attr.nil?
-        store(value.name, value)
-      elsif old_attr.kind_of? Hash
-        old_attr[value.prefix] = value
-      elsif old_attr.prefix != value.prefix
-        store value.name, {old_attr.prefix => old_attr,
-                           value.prefix    => value}
-      else
-        store value.name, value
-      end
+      store name, value
       @element
     end
 
@@ -2398,20 +2355,7 @@ def []=( name, value )
     #   d.root.attributes.prefixes # => ["x", "y"]
     #
     def prefixes
-      ns = []
-      each_attribute do |attribute|
-        ns << attribute.name if attribute.prefix == 'xmlns'
-      end
-      doctype = @element.document&.doctype
-      if doctype
-        expn = @element.expanded_name
-        expn = doctype.name if expn.size == 0
-        doctype.attributes_of(expn).each {
-          |attribute|
-          ns << attribute.name if attribute.prefix == 'xmlns'
-        }
-      end
-      ns
+      namespaces.keys - ['xmlns']
     end
 
     # :call-seq:
@@ -2425,17 +2369,8 @@ def prefixes
     #
     def namespaces
       namespaces = {}
-      each_attribute do |attribute|
-        namespaces[attribute.name] = attribute.value if attribute.prefix == 'xmlns' or attribute.name == 'xmlns'
-      end
-      doctype = @element.document&.doctype
-      if doctype
-        expn = @element.expanded_name
-        expn = doctype.name if expn.size == 0
-        doctype.attributes_of(expn).each {
-          |attribute|
-          namespaces[attribute.name] = attribute.value if attribute.prefix == 'xmlns' or attribute.name == 'xmlns'
-        }
+      each_effective_attribute do |attribute|
+        namespaces[attribute.name] = attribute.value if attribute.namespace_declaration?
       end
       namespaces
     end
@@ -2468,28 +2403,11 @@ def namespaces
     #   attrs.delete(attr) # => <ele att='&lt;'/> # => <ele att='&lt;'/>
     #   attrs.delete(attr) # => <ele att='&lt;'/> # => <ele/>
     #
-    def delete( attribute )
-      name = nil
-      prefix = nil
-      if attribute.kind_of? Attribute
-        name = attribute.name
-        prefix = attribute.prefix
-      else
-        attribute =~ Namespace::NAMESPLIT
-        prefix, name = $1, $2
-        prefix = '' unless prefix
-      end
-      old = fetch(name, nil)
-      if old.kind_of? Hash # the supplied attribute is one of many
-        old.delete(prefix)
-        if old.size == 1
-          repl = nil
-          old.each_value{|v| repl = v}
-          store name, repl
-        end
-      elsif old # the supplied attribute is a top-level one
-        super(name)
-      end
+    def delete(attribute_or_name)
+      key = attribute_or_name
+      key = attribute_or_name.expanded_name if attribute_or_name.kind_of? Attribute
+      super(key)
+
       @element
     end
 
@@ -2514,7 +2432,7 @@ def delete( attribute )
     #   attrs.include?('baz') # => true
     #
     def add( attribute )
-      self[attribute.name] = attribute
+      self[attribute.expanded_name] = attribute
     end
 
     alias :<< :add
@@ -2527,21 +2445,26 @@ def add( attribute )
     #
     #   xml_string = <<-EOT
     #     <root xmlns:foo="http://foo" xmlns:bar="http://bar">
-    #        <ele foo:att='1' bar:att='2' att='&lt;'/>
+    #        <ele foo:att='1' att='&lt;' foo:other='2' other='3'/>
     #     </root>
     #   EOT
     #   d = REXML::Document.new(xml_string)
-    #   ele = d.root.elements['//ele'] # => <a foo:att='1' bar:att='2' att='&lt;'/>
+    #   ele = d.root.elements['//ele'] # => <a foo:att='1' att='&lt;' foo:other='2' other='3'/>
     #   attrs = ele.attributes
-    #   attrs.delete_all('att') # => [att='&lt;']
+    #   attrs.delete_all('att') # => [foo:att='1', att='&lt;']
+    #   attrs.each_attribute.map(&:expanded_name) #=> ['foo:other', 'other']
     #
     def delete_all( name )
-      rv = []
-      each_attribute { |attribute|
-        rv << attribute if attribute.expanded_name == name
-      }
-      rv.each{ |attr| attr.remove }
-      rv
+      attributes = each_attribute.select do |attribute|
+        # For <element xmlns:foo="url" ns:foo="value"/>
+        # delete_all('foo') should not delete xmlns:foo="url"
+        # because it is a namespace declaration, not a normal attribute.
+        (!attribute.namespace_declaration? && attribute.name == name) || attribute.expanded_name == name
+      end
+      attributes.each do |attribute|
+        delete attribute.expanded_name
+      end
+      attributes
     end
 
     # :call-seq:
@@ -2562,17 +2485,51 @@ def delete_all( name )
     #   attrs.get_attribute_ns('http://foo', 'nosuch') # => nil
     #
     def get_attribute_ns(namespace, name)
-      result = nil
-      each_attribute() { |attribute|
-        if name == attribute.name &&
-          namespace == attribute.namespace() &&
-          ( !namespace.empty? || !attribute.fully_expanded_name.index(':') )
-          # foo will match xmlns:foo, but only if foo isn't also an attribute
-          result = attribute if !result or !namespace.empty? or
-                                !attribute.fully_expanded_name.index(':')
+      each_effective_attribute.find do |attribute|
+        if attribute.namespace_declaration?
+          # namespace declarations are not considered as attributes in this method.
+          # For example: <elem1 xmlns="" xmlns:foo="url" /><elem2 xmlns="bar" xmlns:foo="url" />
+          # Both elem1.get_attribute_ns('', 'foo') and elem2.get_attribute_ns('bar', 'foo')
+          # should not match.
+          false
+        elsif namespace.empty? && !attribute.prefix.empty?
+          # If prefix is present, namespace url shouldn't be empty, so it never matches.
+          # For example, <elem foo:att="value" />, even if attribute.namespace returns '',
+          # it just means that namespace lookup failed, it shouldn't match.
+          false
+        else
+          name == attribute.name && namespace == attribute.namespace()
         end
-      }
-      result
+      end
+    end
+
+    private
+
+    # Iterates over the attributes of the element and those in the DTD.
+    # If the element has an attribute with the same name as one in the DTD,
+    # the element's attribute takes precedence.
+    def each_effective_attribute
+      return to_enum(__method__) unless block_given?
+      attr = attlist_attributes
+      attr = attr ? attr.merge(self) : self
+      attr.each_value do |attribute|
+        yield attribute
+      end
+    end
+
+    alias each_own_attribute each_value
+    private :each_own_attribute
+
+    def attlist_attributes
+      doctype = @element.document&.doctype
+      if doctype
+        expn = @element.is_a?(Document) ? doctype.name : @element.expanded_name
+        doctype.attribute_declarations_of(expn).map do |key, value|
+          attr = Attribute.new(key, value)
+          attr.element = @element
+          [key, attr]
+        end.to_h
+      end
     end
   end
 end
diff --git a/test/test_attributes.rb b/test/test_attributes.rb
index 09fde442..af42f06f 100644
--- a/test/test_attributes.rb
+++ b/test/test_attributes.rb
@@ -74,6 +74,16 @@ def test_delete
       assert_equal '4', doc.root.attributes['z:foo']
     end
 
+    def test_delete_all
+      doc = Document.new("<a xmlns:x='x' xmlns:y='y' x:x='0' y:x='1' x='2' y='3' x:y='4' x:z='5' y:y='6'/>")
+      doc.root.attributes.delete_all 'x'
+      assert_equal ['xmlns:x', 'xmlns:y', 'y', 'x:y', 'x:z', 'y:y'], doc.root.attributes.each_attribute.map(&:expanded_name)
+      doc.root.attributes.delete_all 'x:y'
+      assert_equal ['xmlns:x', 'xmlns:y', 'y', 'x:z', 'y:y'], doc.root.attributes.each_attribute.map(&:expanded_name)
+      doc.root.attributes.delete_all 'xmlns:y'
+      assert_equal ['xmlns:x', 'y', 'x:z', 'y:y'], doc.root.attributes.each_attribute.map(&:expanded_name)
+    end
+
     def test_prefixes
       doc = Document.new("<a xmlns='foo' xmlns:x='bar' xmlns:y='twee' z='glorp' x:k='gru'/>")
       prefixes = doc.root.attributes.prefixes
diff --git a/test/test_attributes_mixin.rb b/test/test_attributes_mixin.rb
index 2b9108cb..e4d5841c 100644
--- a/test/test_attributes_mixin.rb
+++ b/test/test_attributes_mixin.rb
@@ -5,8 +5,10 @@ class TestAttributes < Test::Unit::TestCase
     def setup
       @ns_a = "urn:x-test:a"
       @ns_b = "urn:x-test:b"
+      @ns_default = "urn:x-test:default"
       element_string = <<-"XMLEND"
-      <test xmlns:a="#{@ns_a}"
+      <test xmlns="#{@ns_default}"
+            xmlns:a="#{@ns_a}"
             xmlns:b="#{@ns_b}"
             a = "1"
             b = '2'
@@ -25,6 +27,8 @@ def test_get_attribute_ns
       assert_equal("4", @attributes.get_attribute_ns(@ns_a, "d").value)
       assert_equal("5", @attributes.get_attribute_ns(@ns_a, "e").value)
       assert_equal("6", @attributes.get_attribute_ns(@ns_b, "f").value)
+      assert_nil(@attributes.get_attribute_ns(@ns_default, "a"))
+      assert_nil(@attributes.get_attribute_ns("", "xmlns"))
     end
   end
 end
diff --git a/test/test_contrib.rb b/test/test_contrib.rb
index 0b66672c..2a538f52 100644
--- a/test/test_contrib.rb
+++ b/test/test_contrib.rb
@@ -286,10 +286,10 @@ def test_element_cloning_namespace_Chris
       aDoc = REXML::Document.new '<h1 tpl:content="title" xmlns:tpl="1">Dummy title</h1>'
 
       anElement = anElement = aDoc.elements[1]
-      elementAttrPrefix = anElement.attributes.get_attribute('content').prefix
+      elementAttrPrefix = anElement.attributes.get_attribute('tpl:content').prefix
 
       aClone = anElement.clone
-      cloneAttrPrefix = aClone.attributes.get_attribute('content').prefix
+      cloneAttrPrefix = aClone.attributes.get_attribute('tpl:content').prefix
 
       assert_equal( elementAttrPrefix , cloneAttrPrefix )
     end
diff --git a/test/test_core.rb b/test/test_core.rb
index beaaeed7..d4962507 100644
--- a/test/test_core.rb
+++ b/test/test_core.rb
@@ -889,16 +889,25 @@ def test_attlist_decl
       <!ATTLIST a
         bar          CDATA "gobble"
         xmlns:one    CDATA  "two"
+        one:baz      CDATA "three"
       >
       ]>
       <a xmlns:three='xxx' three='yyy'><one:b/><three:c/></a>
       EOL
       assert_equal '<!ATTLIST blah xmlns CDATA "foo">', doc.doctype.children[0].to_s.gsub(/\s+/, " ")
       assert_equal 'gobble', doc.root.attributes['bar']
+      assert_equal 'gobble', doc.root.attributes.get_attribute('bar').value
+      assert_equal 'three', doc.root.attributes.get_attribute('one:baz').value
+      assert_equal 'three', doc.root.attributes.get_attribute_ns('two', 'baz').value
       assert_equal 'xxx', doc.root.elements[2].namespace
       assert_equal 'two', doc.root.elements[1].namespace
       assert_equal 'foo', doc.root.namespace
 
+      # Not used in REXML internal any more, keep for backward compatibility
+      assert_equal 'gobble', doc.doctype.attribute_of('a', 'bar')
+      expected_attributes = [Attribute.new('bar', 'gobble'), Attribute.new('xmlns:one', 'two'), Attribute.new('one:baz', 'three')]
+      assert_equal expected_attributes, doc.doctype.attributes_of('a')
+
       doc = Document.new <<~EOL
       <?xml version="1.0"?>
       <!DOCTYPE schema SYSTEM "XMLSchema.dtd" [
@@ -1537,12 +1546,10 @@ def test_ticket_138
            'inkscape:version="0.44" version="1.0"/>'
       )
       expected = {
-        "inkscape" => attribute("xmlns:inkscape",
+        "xmlns:inkscape" => attribute("xmlns:inkscape",
                                 "http://www.inkscape.org/namespaces/inkscape"),
-        "version" => {
-          "inkscape" => attribute("inkscape:version", "0.44"),
-          "" => attribute("version", "1.0"),
-        },
+        "inkscape:version" => attribute("inkscape:version", "0.44"),
+        "version" => attribute("version", "1.0"),
       }
       assert_equal(expected,
                    doc.root.attributes.to_h)