Whitespace inside nested block elements in conditional comments should be ignored.

When content inside a conditional comment is formatted with indentation and
newlines, html5lib parses these as TextNode children of the elements.
For block-level elements like table/tr/td, this whitespace is semantically
insignificant and should be ignored.

Author: FelixSchwarz

htmlcompare/elements.py

@@ -25,6 +25,12 @@
     'h1', 'h2', 'h3', 'h4', 'h5', 'h6',
     # Metadata (typically not rendered)
     'base', 'link', 'meta', 'noscript', 'script', 'style', 'template', 'title',
+    # VML elements (used in Outlook conditional comments)
+    'v:image', 'v:rect', 'v:fill', 'v:stroke', 'v:textbox', 'v:shape',
+    'v:shapetype', 'v:roundrect', 'v:oval', 'v:line', 'v:polyline',
+    'v:group', 'v:background',
+    # Microsoft Office elements
+    'o:p', 'o:wrapblock',
 })
 
 # Inline elements where whitespace is significant for rendering.

htmlcompare/tests/compare_test.py

@@ -625,3 +625,23 @@ def test_style_tag_detects_different_media_query_content():
         '<style>@media screen { .foo { width: 50%; } }</style>',
     )
     assert not result.is_equal
+
+
+# --- Conditional Comment Tests ---
+
+def test_conditional_comment_ignores_whitespace_inside_nested_block_elements():
+    """Whitespace inside nested block elements in conditional comments should be ignored.
+
+    When content inside a conditional comment is formatted with indentation and newlines,
+    html5lib parses these as TextNode children of the elements. For block-level elements
+    like table/tr/td, this whitespace is semantically insignificant and should be ignored.
+    """
+    multi_line = '''<div><!--[if mso | IE]>
+                <table>
+                    <tr>
+                        <td>
+                            <v:image src="test.jpg" />
+            <![endif]--></div>'''
+    single_line = '<div><!--[if mso | IE]><table><tr><td><v:image src="test.jpg" /><![endif]--></div>'  # noqa: E501
+    result = compare_html(multi_line, single_line)
+    assert result.is_equal