diff --git a/docs/changelog.md b/docs/changelog.md
index 5dc7b0f8..44dcf618 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -27,6 +27,10 @@ See the [Contributing Guide](contributing.md) for details.
   and closing backticks did not match (#1668).
 * Escape the language name in the `codehilite` extension's non-Pygments output to
   maintain consistency with Pygments output.
+* Preserve attributes without a value in the `attr_list` extension exactly as
+  provided, rather than rendering them as `key="key"`, so the same source
+  renders consistently under both the `html` and `xhtml` output formats
+  (#1501).
 
 ## [3.11.2] - 2026-10-19
 
diff --git a/markdown/extensions/attr_list.py b/markdown/extensions/attr_list.py
index b3b8d951..060e4761 100644
--- a/markdown/extensions/attr_list.py
+++ b/markdown/extensions/attr_list.py
@@ -28,6 +28,7 @@
 from typing import TYPE_CHECKING
 
 from . import Extension
+from ..serializers import BOOLEAN_ATTRIBUTES
 from ..treeprocessors import Treeprocessor
 import re
 
@@ -54,7 +55,12 @@ def _handle_word(s, t):
         return '.', t[1:]
     if t.startswith('#'):
         return 'id', t[1:]
-    return t, t
+    # A word without a value is a valueless attribute. It is represented
+    # internally with a value of `None` so that what the user provided can
+    # be preserved. Consumers which assign the attribute to an element are
+    # responsible for restoring the historic `key="key"` representation
+    # for known boolean attributes (see `AttrListTreeprocessor.assign_attrs`).
+    return t, None
 
 
 _scanner = re.Scanner([
@@ -66,9 +72,11 @@ def _handle_word(s, t):
 ])
 
 
-def get_attrs_and_remainder(attrs_string: str) -> tuple[list[tuple[str, str]], str]:
+def get_attrs_and_remainder(attrs_string: str) -> tuple[list[tuple[str, str | None]], str]:
     """ Parse attribute list and return a list of attribute tuples.
 
+    Attributes provided without a value are returned with a value of `None`.
+
     Additionally, return any text that remained after a curly brace. In typical cases, its presence
     should mean that the input does not match the intended attribute list syntax.
     """
@@ -79,7 +87,7 @@ def get_attrs_and_remainder(attrs_string: str) -> tuple[list[tuple[str, str]], s
     return attrs, remainder
 
 
-def get_attrs(str: str) -> list[tuple[str, str]]:  # pragma: no cover
+def get_attrs(str: str) -> list[tuple[str, str | None]]:  # pragma: no cover
     """ Soft-deprecated. Prefer `get_attrs_and_remainder`. """
     return get_attrs_and_remainder(str)[0]
 
@@ -190,6 +198,18 @@ def assign_attrs(self, elem: Element, attrs_string: str, *, strict: bool = False
                     elem.set('class', '{} {}'.format(cls, v))
                 else:
                     elem.set('class', v)
+            elif v is None:
+                # assign a valueless attribute. Maintain the historic
+                # `key="key"` representation only for known boolean
+                # attributes (which differ per output format). For any
+                # other attribute, preserve what the user provided by
+                # keeping the value as `None`, which the serializer
+                # renders as the bare attribute name.
+                name = self.sanitize_name(k)
+                if name.lower() in BOOLEAN_ATTRIBUTES.get(self.md.output_format, frozenset()):
+                    elem.set(name, name)
+                else:
+                    elem.set(name, None)
             else:
                 # assign attribute `k` with `v`
                 elem.set(self.sanitize_name(k), v)
diff --git a/markdown/extensions/fenced_code.py b/markdown/extensions/fenced_code.py
index fbdd015b..6e2ed6b1 100644
--- a/markdown/extensions/fenced_code.py
+++ b/markdown/extensions/fenced_code.py
@@ -171,12 +171,18 @@ def run(self, lines: list[str]) -> list[str]:
                 break
         return text.split("\n")
 
-    def handle_attrs(self, attrs: Iterable[tuple[str, str]]) -> tuple[str, list[str], dict[str, Any]]:
+    def handle_attrs(
+        self, attrs: Iterable[tuple[str, str | None]]
+    ) -> tuple[str, list[str], dict[str, Any]]:
         """ Return tuple: `(id, [list, of, classes], {configs})` """
         id = ''
         classes = []
         configs = {}
         for k, v in attrs:
+            if v is None:
+                # A valueless attribute is a flag: preserve the
+                # historic behavior of using the key as its own value.
+                v = k
             if k == 'id':
                 id = v
             elif k == '.':
diff --git a/markdown/serializers.py b/markdown/serializers.py
index 255092b4..69f031cc 100644
--- a/markdown/serializers.py
+++ b/markdown/serializers.py
@@ -55,6 +55,27 @@
 
 RE_AMP = re.compile(r'&(?!(?:\#[0-9]+|\#x[0-9a-f]+|[0-9a-z]+);)', re.I)
 
+# Known boolean attributes, whose presence alone carries the meaning and
+# which may therefore be rendered in the minimized (valueless) form. The
+# list of recognized attributes differs per output format: `xhtml` only
+# recognizes the attributes declared boolean by the XHTML 1.0 spec, while
+# `html` recognizes the boolean attributes of the HTML spec. Note that
+# `download` is deliberately absent from both lists: it is only boolean
+# when no value is assigned and a value changes its meaning.
+BOOLEAN_ATTRIBUTES: dict[str, frozenset[str]] = {
+    'html': frozenset([
+        'allowfullscreen', 'async', 'autofocus', 'autoplay', 'checked',
+        'controls', 'default', 'defer', 'disabled', 'formnovalidate',
+        'hidden', 'inert', 'ismap', 'itemscope', 'loop', 'multiple', 'muted',
+        'nomodule', 'novalidate', 'open', 'playsinline', 'readonly',
+        'required', 'reversed', 'selected',
+    ]),
+    'xhtml': frozenset([
+        'checked', 'compact', 'declare', 'defer', 'disabled', 'ismap',
+        'multiple', 'noresize', 'noshade', 'nowrap', 'readonly', 'selected',
+    ]),
+}
+
 
 def _raise_serialization_error(text: str) -> NoReturn:  # pragma: no cover
     raise TypeError(
@@ -162,13 +183,17 @@ def _serialize_html(write: Callable[[str], None], elem: Element, format: Literal
                     if isinstance(v, QName):
                         # Assume a text only `QName`
                         v = v.text
+                    if v is None:
+                        # A valueless attribute is preserved exactly as it
+                        # was provided: render the bare attribute name.
+                        write(f" {k}")
                     else:
                         v = _escape_attrib_html(v)
-                    if k == v and format == 'html':
-                        # handle boolean attributes
-                        write(f" {v}")
-                    else:
-                        write(f' {k}="{v}"')
+                        if k == v and format == 'html' and k in BOOLEAN_ATTRIBUTES['html']:
+                            # handle boolean attributes
+                            write(f" {v}")
+                        else:
+                            write(f' {k}="{v}"')
             if namespace_uri:
                 write(f' xmlns="{_escape_attrib(namespace_uri)}"')
             if format == "xhtml" and tag.lower() in HTML_EMPTY:
diff --git a/markdown/treeprocessors.py b/markdown/treeprocessors.py
index a148175f..63904b39 100644
--- a/markdown/treeprocessors.py
+++ b/markdown/treeprocessors.py
@@ -480,4 +480,7 @@ def run(self, root: etree.Element) -> None:
                 elem.tail = self.unescape(elem.tail)
             # Unescape attribute values
             for key, value in elem.items():
-                elem.set(key, self.unescape(value))
+                if value is not None:
+                    # A value of `None` is a valueless attribute, which
+                    # has nothing to unescape.
+                    elem.set(key, self.unescape(value))
diff --git a/tests/extensions/attr_list.html b/tests/extensions/attr_list.html
index e7bfe0bf..600934c1 100644
--- a/tests/extensions/attr_list.html
+++ b/tests/extensions/attr_list.html
@@ -6,7 +6,6 @@ <h2 class="someclass" id="sextext2">This is another</h2>
 A <a class="linkkyclass" href="http://example.com" title="A title.">link</a>
 And a <strong class="nest">nested <a class="linky2" href="http://example.com" title="Some title">link</a></strong></p>
 <h3 id="hash">This is a hash Header</h3>
-<p bar="b az" baz="blah blah" foo="foo" title="I wasn't kidding!">And now some random attributes.</p>
 <h3 id="hash2">No closing hash header</h3>
 <p class="foo bar addme" id="overridden">Now test overrides</p>
 <pre><code># A code block which contains attr_list syntax
@@ -15,7 +14,6 @@ <h3 id="hash2">No closing hash header</h3>
 </code></pre>
 <h3 id="hash3">No colon for compatibility with Headerid ext</h3>
 <p id="the_end">Also a codespan: <code class="foo">{: .someclass}</code>.</p>
-<h3 _:="{:" id="hash5">Bad Syntax</h3>
 <ul>
 <li class="item">Item1</li>
 <li class="item">Item2<ul>
@@ -59,7 +57,6 @@ <h1 class="block">Definition <em>lists</em></h1>
 <dd>Some dd</dd>
 </dl>
 <h1>Bad attributes</h1>
-<p>Key without <em foo="foo">value</em></p>
 <p>Value without <em>key</em></p>
 <p>No <em>key or value</em></p>
 <p><em>Weirdness</em></p>
diff --git a/tests/extensions/attr_list.txt b/tests/extensions/attr_list.txt
index 465ce4f5..1d2a9272 100644
--- a/tests/extensions/attr_list.txt
+++ b/tests/extensions/attr_list.txt
@@ -16,9 +16,6 @@ And a __nested [link][]{: .linky2}__{: .nest}
 
 ### This is a hash Header ### {: #hash}
 
-And now some random attributes.
-{:foo bar='b az' baz="blah blah" title="I wasn't kidding!" }
-
 ### No closing hash header {: #hash2}
 
 Now test overrides
@@ -33,8 +30,6 @@ Now test overrides
 Also a codespan: `{: .someclass}`{: .foo}.
 {: #the_end}
 
-### Bad Syntax { {: #hash5 }
-
 * Item1
   {: .item }
 * Item2
@@ -78,8 +73,6 @@ DT2 {.term}
 
 # Bad attributes
 
-Key without *value*{ foo= }
-
 Value without *key*{ =bar }
 
 No *key or value*{ = }
diff --git a/tests/test_syntax/extensions/test_attr_list.py b/tests/test_syntax/extensions/test_attr_list.py
index dabac77d..73c77969 100644
--- a/tests/test_syntax/extensions/test_attr_list.py
+++ b/tests/test_syntax/extensions/test_attr_list.py
@@ -133,3 +133,171 @@ def test_table_td(self):
             ),
             extensions=['attr_list', 'tables']
         )
+
+
+class TestAttrListValuelessAttributes(TestCase):
+    """ Valueless (boolean-style) attributes are preserved as provided.
+
+    Only known boolean attributes keep the historic `key="key"`
+    representation, and the list of known attributes differs per
+    output format. See issue #1501.
+    """
+
+    maxDiff = None
+    default_kwargs = {'extensions': ['attr_list']}
+
+    def test_valueless_non_boolean_preserved_xhtml(self):
+        self.assertMarkdownRenders(
+            '[download](file.txt){download}',
+            '<p><a download href="file.txt">download</a></p>'
+        )
+
+    def test_valueless_non_boolean_preserved_html(self):
+        self.assertMarkdownRenders(
+            '[download](file.txt){download}',
+            '<p><a download href="file.txt">download</a></p>',
+            output_format='html'
+        )
+
+    def test_valueless_non_boolean_consistent_across_formats(self):
+        # Previously `xhtml` rendered `download="download"` (which gives the
+        # downloaded file a default name) while `html` rendered bare
+        # `download` (which does not): the same source changed meaning with
+        # the output format.
+        for output_format in ['xhtml', 'html']:
+            self.assertMarkdownRenders(
+                '[download](file.txt){download}',
+                '<p><a download href="file.txt">download</a></p>',
+                output_format=output_format
+            )
+
+    def test_explicit_value_equal_to_name_preserved(self):
+        # An explicitly assigned value must not be dropped, even when it is
+        # equal to the attribute name and the attribute is not boolean.
+        for output_format in ['xhtml', 'html']:
+            self.assertMarkdownRenders(
+                '[download](file.txt){download="download"}',
+                '<p><a download="download" href="file.txt">download</a></p>',
+                output_format=output_format
+            )
+
+    def test_known_boolean_attribute_xhtml(self):
+        self.assertMarkdownRenders(
+            '[x](y){checked}',
+            '<p><a checked="checked" href="y">x</a></p>'
+        )
+
+    def test_known_boolean_attribute_html(self):
+        self.assertMarkdownRenders(
+            '[x](y){checked}',
+            '<p><a checked href="y">x</a></p>',
+            output_format='html'
+        )
+
+    def test_xhtml_only_boolean_attribute(self):
+        # `compact` is a boolean attribute in XHTML 1.0 but not in HTML.
+        self.assertMarkdownRenders(
+            '# Heading {compact}',
+            '<h1 compact="compact">Heading</h1>'
+        )
+        self.assertMarkdownRenders(
+            '# Heading {compact}',
+            '<h1 compact>Heading</h1>',
+            output_format='html'
+        )
+
+    def test_html_only_boolean_attribute(self):
+        # `open` is a boolean attribute in HTML but not in XHTML 1.0. It
+        # renders bare in both formats: in HTML as a known boolean, and in
+        # XHTML as a valueless attribute preserved as provided.
+        for output_format in ['xhtml', 'html']:
+            self.assertMarkdownRenders(
+                '[x](y){open}',
+                '<p><a href="y" open>x</a></p>',
+                output_format=output_format
+            )
+
+    def test_generic_valueless_attribute(self):
+        for output_format in ['xhtml', 'html']:
+            self.assertMarkdownRenders(
+                '[x](y){foo}',
+                '<p><a foo href="y">x</a></p>',
+                output_format=output_format
+            )
+
+    def test_valueless_attribute_on_block_element(self):
+        self.assertMarkdownRenders(
+            '# Heading {download}',
+            '<h1 download>Heading</h1>'
+        )
+
+    def test_get_attrs_and_remainder_marks_valueless_as_none(self):
+        from markdown.extensions.attr_list import get_attrs_and_remainder
+        attrs, remainder = get_attrs_and_remainder('download checked key=value .cls #the-id')
+        self.assertEqual(remainder, '')
+        self.assertIn(('download', None), attrs)
+        self.assertIn(('checked', None), attrs)
+        self.assertIn(['key', 'value'], attrs)
+        self.assertIn(('.', 'cls'), attrs)
+        self.assertIn(('id', 'the-id'), attrs)
+
+    def test_serializer_renders_none_value_as_bare_name(self):
+        import xml.etree.ElementTree as etree
+
+        from markdown.serializers import to_html_string, to_xhtml_string
+
+        elem = etree.Element('a')
+        elem.set('download', None)
+        elem.set('checked', 'checked')
+        elem.set('href', 'file.txt')
+        elem.text = 'x'
+        self.assertEqual(
+            to_html_string(elem),
+            '<a checked download href="file.txt">x</a>'
+        )
+        self.assertEqual(
+            to_xhtml_string(elem),
+            '<a checked="checked" download href="file.txt">x</a>'
+        )
+
+    def test_serializer_preserves_explicit_value_equal_to_name(self):
+        import xml.etree.ElementTree as etree
+
+        from markdown.serializers import to_html_string, to_xhtml_string
+
+        elem = etree.Element('a')
+        elem.set('download', 'download')
+        elem.text = 'x'
+        self.assertEqual(to_html_string(elem), '<a download="download">x</a>')
+        self.assertEqual(to_xhtml_string(elem), '<a download="download">x</a>')
+
+    def test_fenced_code_valueless_word(self):
+        # `fenced_code` shares the attribute parser: a valueless word
+        # keeps its historic treatment there (key used as its own value).
+        self.assertMarkdownRenders(
+            '``` { .python linenos }\nx = 1\n```',
+            '<pre><code class="language-python" linenos="linenos">x = 1\n</code></pre>',
+            extensions=['fenced_code', 'attr_list']
+        )
+
+    # The next three tests were moved here from the legacy fixture tests
+    # in `tests/extensions/attr_list.txt` because their expected output
+    # changed with the preservation of valueless attributes.
+
+    def test_valueless_attribute_among_valued_attributes(self):
+        self.assertMarkdownRenders(
+            "And now some random attributes.\n{:foo bar='b az' baz=\"blah blah\" title=\"I wasn't kidding!\" }",
+            '<p bar="b az" baz="blah blah" foo title="I wasn\'t kidding!">And now some random attributes.</p>'
+        )
+
+    def test_bad_syntax_header_with_valueless_attribute(self):
+        self.assertMarkdownRenders(
+            '### Bad Syntax { {: #hash5 }',
+            '<h3 _: id="hash5">Bad Syntax</h3>'
+        )
+
+    def test_key_without_value(self):
+        self.assertMarkdownRenders(
+            'Key without *value*{ foo= }',
+            '<p>Key without <em foo>value</em></p>'
+        )