Bug: Strings in Excel number fomat do not preserve case(fixes #6317)

pandas/io/formats/css.py

@@ -20,6 +20,22 @@
     )
 
 
+def _normalize_number_format_value(value: str) -> str:
+    value = value.strip()
+    out: list[str] = []
+    in_string = False
+
+    for ch in value:
+        if ch == '"':
+            out.append(ch)
+            in_string = not in_string
+        elif in_string:
+            out.append(ch)  # preserve case inside string literals
+        else:
+            out.append(ch.lower())  # normalize outside literals
+    return "".join(out)
+
+
 def _side_expander(prop_fmt: str) -> Callable:
     """
     Wrapper to expand shorthand property into top, right, bottom, left properties
@@ -391,7 +407,10 @@ def _error() -> str:
     def atomize(self, declarations: Iterable) -> Generator[tuple[str, str]]:
         for prop, value in declarations:
             prop = prop.lower()
-            value = value.lower()
+            if prop == "number-format":
+                value = _normalize_number_format_value(value)
+            else:
+                value = value.lower()
             if prop in self.CSS_EXPANSIONS:
                 expand = self.CSS_EXPANSIONS[prop]
                 yield from expand(self, prop, value)
@@ -414,7 +433,10 @@ def parse(self, declarations_str: str) -> Iterator[tuple[str, str]]:
             prop, sep, val = decl.partition(":")
             prop = prop.strip().lower()
             # TODO: don't lowercase case sensitive parts of values (strings)
-            val = val.strip().lower()
+            if prop == "number-format":
+                val = _normalize_number_format_value(val)
+            else:
+                val = val.strip().lower()
             if sep:
                 yield prop, val
             else:

pandas/tests/io/excel/test_style.py

@@ -96,6 +96,7 @@ def test_styler_to_excel_unstyled(engine, tmp_excel):
     ("font-style: italic;", ["font", "i"], True),
     ("text-decoration: underline;", ["font", "u"], "single"),
     ("number-format: $??,???.00;", ["number_format"], "$??,???.00"),
+    ('number-format: #,,"M";', ["number_format"], '#,,"M"'),
     ("text-align: left;", ["alignment", "horizontal"], "left"),
     (
         "vertical-align: bottom;",

pandas/tests/io/formats/test_css.py

@@ -286,3 +286,23 @@ def test_css_relative_font_size(size, relative_to, resolved):
     else:
         inherited = {"font-size": relative_to}
     assert_resolves(f"font-size: {size}", {"font-size": resolved}, inherited=inherited)
+
+
+def test_css_atomize_preserves_number_format_case():
+    resolver = CSSResolver()
+    declarations = [("NUMBER-FORMAT", '#,,"M"')]
+
+    props = dict(resolver.atomize(declarations))
+
+    assert "number-format" in props
+    assert props["number-format"] == '#,,"M"'
+
+
+def test_css_parse_preserves_number_format_case():
+    resolver = CSSResolver()
+
+    css = 'NUMBER-FORMAT: #,,"M"; COLOR: Red'
+    result = dict(resolver.parse(css))
+
+    assert result["number-format"] == '#,,"M"'
+    assert result["color"] == "red"