Ensure incomplete markup declaration in raw HTML doesn't crash parser.

docs/changelog.md

@@ -13,6 +13,7 @@ See the [Contributing Guide](contributing.md) for details.
 
 ### Fixed
 
+* Ensure incomplete markup declaration in raw HTML doesn't crash parser (#1534).
 * Fixed dropped content in `md_in_html` (#1526).
 * Fixed HTML handling corner case that prevented some content from not being rendered (#1528).
 

markdown/extensions/md_in_html.py

@@ -280,6 +280,10 @@ def parse_pi(self, i: int) -> int:
 
     def parse_html_declaration(self, i: int) -> int:
         if self.at_line_start() or self.intail or self.mdstack:
+            if self.rawdata[i:i+3] == '<![' and not self.rawdata[i:i+9] == '<![CDATA[':
+                # We have encountered the bug in #1534 (Python bug `gh-77057`).
+                # Provide an override until we drop support for Python < 3.13.
+                return self.parse_bogus_comment(i)
             # The same override exists in `HTMLExtractor` without the check
             # for `mdstack`. Therefore, use parent of `HTMLExtractor` instead.
             return super(HTMLExtractor, self).parse_html_declaration(i)

markdown/htmlparser.py

@@ -275,6 +275,10 @@ def parse_pi(self, i: int) -> int:
 
     def parse_html_declaration(self, i: int) -> int:
         if self.at_line_start() or self.intail:
+            if self.rawdata[i:i+3] == '<![' and not self.rawdata[i:i+9] == '<![CDATA[':
+                # We have encountered the bug in #1534 (Python bug `gh-77057`).
+                # Provide an override until we drop support for Python < 3.13.
+                return self.parse_bogus_comment(i)
             return super().parse_html_declaration(i)
         # This is not the beginning of a raw block so treat as plain data
         # and avoid consuming any tags which may follow (see #1066).

tests/test_syntax/blocks/test_html_blocks.py

@@ -1275,6 +1275,13 @@ def test_raw_cdata_indented(self):
             )
         )
 
+    def test_not_actually_cdata(self):
+        # Ensure bug reported in #1534 is avoided.
+        self.assertMarkdownRenders(
+            '<![',
+            '<p>&lt;![</p>'
+        )
+
     def test_raw_cdata_code_span(self):
         self.assertMarkdownRenders(
             self.dedent(