diff --git a/Lib/html/parser.py b/Lib/html/parser.py
index 80fb8c3f929f6b..ee1189f95ca200 100644
--- a/Lib/html/parser.py
+++ b/Lib/html/parser.py
@@ -8,6 +8,7 @@
 # and CDATA (character data -- only end tags are special).
 
 
+import string
 import re
 import _markupbase
 
@@ -105,6 +106,10 @@ def _replace_attr_charref(match):
 def _unescape_attrvalue(s):
     return attr_charref.sub(_replace_attr_charref, s)
 
+def _ascii_lower(s, *, table=str.maketrans(string.ascii_uppercase,
+                                           string.ascii_lowercase)):
+    return s.translate(table)
+
 
 class HTMLParser(_markupbase.ParserBase):
     """Find tags and other markup and call handler functions.
@@ -179,7 +184,7 @@ def get_starttag_text(self):
         return self.__starttag_text
 
     def set_cdata_mode(self, elem, *, escapable=False):
-        self.cdata_elem = elem.lower()
+        self.cdata_elem = _ascii_lower(elem)
         self._escapable = escapable
         if self.cdata_elem == 'plaintext':
             self.interesting = re.compile(r'\z')
@@ -284,7 +289,7 @@ def goahead(self, end):
                         self.handle_comment(rawdata[i+4:j])
                     elif startswith("<![CDATA[", i) and self._support_cdata:
                         self.unknown_decl(rawdata[i+3:])
-                    elif rawdata[i:i+9].lower() == '<!doctype':
+                    elif _ascii_lower(rawdata[i:i+9]) == '<!doctype':
                         self.handle_decl(rawdata[i+2:])
                     elif startswith("<!", i):
                         # bogus comment
@@ -372,7 +377,7 @@ def parse_html_declaration(self, i):
                 return -1
             self.unknown_decl(rawdata[i+3: j])
             return j + 3
-        elif rawdata[i:i+9].lower() == '<!doctype':
+        elif _ascii_lower(rawdata[i:i+9]) == '<!doctype':
             # find the closing >
             gtpos = rawdata.find('>', i+9)
             if gtpos == -1:
@@ -438,7 +443,7 @@ def parse_starttag(self, i):
         match = tagfind_tolerant.match(rawdata, i+1)
         assert match, 'unexpected call to parse_starttag()'
         k = match.end()
-        self.lasttag = tag = match.group(1).lower()
+        self.lasttag = tag = _ascii_lower(match.group(1))
         while k < endpos:
             m = attrfind_tolerant.match(rawdata, k)
             if not m:
@@ -451,7 +456,7 @@ def parse_starttag(self, i):
                 attrvalue = attrvalue[1:-1]
             if attrvalue:
                 attrvalue = _unescape_attrvalue(attrvalue)
-            attrs.append((attrname.lower(), attrvalue))
+            attrs.append((_ascii_lower(attrname), attrvalue))
             k = m.end()
 
         end = rawdata[k:endpos].strip()
@@ -507,7 +512,7 @@ def parse_endtag(self, i):
         # https://html.spec.whatwg.org/multipage/parsing.html#tag-name-state
         match = tagfind_tolerant.match(rawdata, i+2)
         assert match
-        tag = match.group(1).lower()
+        tag = _ascii_lower(match.group(1))
         self.handle_endtag(tag)
         self.clear_cdata_mode()
         return j
diff --git a/Lib/test/test_htmlparser.py b/Lib/test/test_htmlparser.py
index e4eff1ea17a670..b061027883a991 100644
--- a/Lib/test/test_htmlparser.py
+++ b/Lib/test/test_htmlparser.py
@@ -533,6 +533,37 @@ def test_invalid_nonascii_closing_tag(self, tag, endtag):
             ("endtag", tag),
         ], collector=EventCollector(convert_charrefs=False, scripting=True))
 
+    @support.subTests('tag,converted', [
+        ('TıTLE', 'tıtle'),
+        ('NOFRAMEſ', 'noframeſ'),
+        ('NOſCRIPT', 'noſcript'),
+        ('NOSCRıPT', 'noscrıpt'),
+        ('SCRıPT', 'scrıpt'),
+        ('ADDREß', 'addreß'),
+        ('DATALIst', 'datalist'),
+        ('Lı', 'lı'),
+        ('LINK', 'linK'),
+    ])
+    def test_nonascii_tag(self, tag, converted):
+        # Starts with ASCII letter
+        source = f"<{tag}><br></{tag}>"
+        self._run_check(source, [
+            ("starttag", converted, []),
+            ("starttag", "br", []),
+            ("endtag", converted),
+        ], collector=EventCollector(convert_charrefs=False, scripting=True))
+
+    @support.subTests('tag', ['ſtyle', 'ſtyle', 'style', 'ıframe', 'ſcript',
+                              'ı', 'KBD', 'ſMALL', 'stRONG'])
+    def test_invalid_nonascii_tag(self, tag):
+        # Starts with non-ASCII letter
+        source = f"<{tag}><br></{tag}>"
+        self._run_check(source, [
+            ("data", f"<{tag}>"),
+            ("starttag", "br", []),
+            ("comment", f"{tag}"),
+        ], collector=EventCollector(convert_charrefs=False, scripting=True))
+
     @support.subTests('tail,end', [
         ('', False),
         ('<', False),
@@ -1068,7 +1099,7 @@ def test_attr_values(self):
             "<a href=mailto:xyz@example.com>",
             [("starttag", "a", [("href", "mailto:xyz@example.com")])])
 
-    def test_attr_nonascii(self):
+    def test_attr_value_nonascii(self):
         # see issue 7311
         self._run_check(
             "<img src=/foo/bar.png alt=\u4e2d\u6587>",
@@ -1083,6 +1114,16 @@ def test_attr_nonascii(self):
             [("starttag", "a", [("title", "\u30c6\u30b9\u30c8"),
                                 ("href", "\u30c6\u30b9\u30c8.html")])])
 
+    def test_attr_name_nonascii(self):
+        self._run_check(
+            '<BUTTON ACCEßKEY="s">',
+            [('starttag', 'button', [('acceßKey', 's')])])
+        self._run_check(
+            '<TRACK KIND="chapters" ſRC="sampleChapters.vtt" SRCLANG="en" />',
+            [('startendtag', 'track', [('Kind', 'chapters'),
+                                       ('ſrc', 'sampleChapters.vtt'),
+                                       ('srclang', 'en')])])
+
     def test_attr_entity_replacement(self):
         self._run_check(
             "<a b='&amp;&gt;&lt;&quot;&apos;'>",
diff --git a/Misc/NEWS.d/next/Library/2025-11-19-19-52-05.gh-issue-141756.7Xcg90.rst b/Misc/NEWS.d/next/Library/2025-11-19-19-52-05.gh-issue-141756.7Xcg90.rst
new file mode 100644
index 00000000000000..0145477cfbf5bd
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-11-19-19-52-05.gh-issue-141756.7Xcg90.rst
@@ -0,0 +1,3 @@
+Fix handling non-ASCII element and attribute names in
+:class:`html.parser.HTMLParser`. Only ASCII letters are now converted to
+lower case.