diff --git a/docs/changelog.md b/docs/changelog.md
index e949c44f..91373276 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -24,6 +24,8 @@ See the [Contributing Guide](contributing.md) for details.
 ### Fixed
 
 * Fix an issue with excessive backtracking when matching inline code blocks (#1617).
+* `md_in_html` now honors tags added to `Markdown.block_level_elements` after
+  the extension is loaded (#1246).
 
 ## [3.10.3] - 2026-07-30
 
diff --git a/markdown/extensions/md_in_html.py b/markdown/extensions/md_in_html.py
index 32a9c11a..d02d7a48 100644
--- a/markdown/extensions/md_in_html.py
+++ b/markdown/extensions/md_in_html.py
@@ -41,8 +41,6 @@ class HTMLExtractorExtra(HTMLExtractor):
     """
 
     def __init__(self, md: Markdown, *args, **kwargs):
-        # All block-level tags.
-        self.block_level_tags = set(md.block_level_elements.copy())
         # Block-level tags in which the content only gets span level parsing
         self.span_tags = set(
             ['address', 'dd', 'dt', 'h1', 'h2', 'h3', 'h4', 'h5', 'h6', 'legend', 'li', 'p', 'summary', 'td', 'th']
@@ -52,9 +50,19 @@ def __init__(self, md: Markdown, *args, **kwargs):
 
         super().__init__(md, *args, **kwargs)
 
-        # Block-level tags in which the content gets parsed as blocks
-        self.block_tags = set(self.block_level_tags) - (self.span_tags | self.raw_tags | self.empty_tags)
-        self.span_and_blocks_tags = self.block_tags | self.span_tags
+    @property
+    def block_level_tags(self) -> set[str]:
+        # Read from the Markdown instance so tags registered after the
+        # extension is loaded (md.block_level_elements.append) are honored.
+        return set(self.md.block_level_elements)
+
+    @property
+    def block_tags(self) -> set[str]:
+        return set(self.block_level_tags) - (self.span_tags | self.raw_tags | self.empty_tags)
+
+    @property
+    def span_and_blocks_tags(self) -> set[str]:
+        return self.block_tags | self.span_tags
 
     def reset(self):
         """Reset this instance.  Loses all unprocessed data."""
diff --git a/tests/test_syntax/extensions/test_md_in_html.py b/tests/test_syntax/extensions/test_md_in_html.py
index 903766fc..c89ae6f8 100644
--- a/tests/test_syntax/extensions/test_md_in_html.py
+++ b/tests/test_syntax/extensions/test_md_in_html.py
@@ -1556,6 +1556,36 @@ def test_noname_tag(self):
             )
         )
 
+    def test_registered_custom_element_is_block_level(self):
+        # Tags added to Markdown.block_level_elements after construction
+        # must still be treated as block-level HTML (#1246).
+        md = Markdown(extensions=['md_in_html'])
+        md.block_level_elements.append('a-b')
+        src = self.dedent(
+            """
+            <a-b>
+
+            asdf
+
+            </a-b>
+            """
+        )
+        self.assertEqual(md.convert(src), '<a-b>\n\nasdf\n\n</a-b>')
+
+    def test_registered_custom_element_markdown_attr(self):
+        md = Markdown(extensions=['md_in_html'])
+        md.block_level_elements.append('a-b')
+        src = self.dedent(
+            """
+            <a-b markdown>
+
+            *asdf*
+
+            </a-b>
+            """
+        )
+        self.assertEqual(md.convert(src), '<a-b>\n<p><em>asdf</em></p>\n</a-b>')
+
 
 def load_tests(loader, tests, pattern):
     """ Ensure `TestHTMLBlocks` doesn't get run twice by excluding it here. """