diff --git a/pandas/io/formats/excel.py b/pandas/io/formats/excel.py
index 5fde6577e9f95..f2cf1a4838c36 100644
--- a/pandas/io/formats/excel.py
+++ b/pandas/io/formats/excel.py
@@ -303,13 +303,13 @@ def _border_style(
         #       'slantDashDot'
         #       'thick'
         #       'thin'
-        if width is None and style is None and color is None:
-            # Return None will remove "border" from style dictionary
-            return None
-
         if width is None and style is None:
-            # Return "none" will keep "border" in style dictionary
-            return "none"
+            if color is None:
+                # Return None will remove "border" from style dictionary
+                return None
+            else:
+                # Return "none" will keep "border" in style dictionary
+                return "none"
 
         if style in ("none", "hidden"):
             return "none"
@@ -411,11 +411,6 @@ def _get_decoration(self, props: Mapping[str, str]) -> Sequence[str]:
         else:
             return ()
 
-    def _get_underline(self, decoration: Sequence[str]) -> str | None:
-        if "underline" in decoration:
-            return "single"
-        return None
-
     def _get_shadow(self, props: Mapping[str, str]) -> bool | None:
         if "text-shadow" in props:
             return bool(re.search("^[^#(]*[1-9]", props["text-shadow"]))
diff --git a/pandas/tests/io/formats/test_to_excel.py b/pandas/tests/io/formats/test_to_excel.py
index b40201b9ba1e6..babbfcb46a6fc 100644
--- a/pandas/tests/io/formats/test_to_excel.py
+++ b/pandas/tests/io/formats/test_to_excel.py
@@ -156,6 +156,14 @@
             "border-top-style: solid; border-top-width: 4pt",
             {"border": {"top": {"style": "thick"}}},
         ),
+        (
+            "border-top-style: solid; border-top-width: none",
+            {"border": {"top": {"style": "none"}}},
+        ),
+        (
+            "border-top-style: solid; border-top-width: 0.000001pt",
+            {"border": {"top": {"style": "none"}}},
+        ),
         (
             "border-top-style: dotted",
             {"border": {"top": {"style": "mediumDashDotDot"}}},
@@ -194,6 +202,10 @@
             "border-top-color: blue",
             {"border": {"top": {"color": "0000FF", "style": "none"}}},
         ),
+        (
+            "border-top-style: slantDashDot; border-top-color: blue",
+            {"border": {"top": {"style": "slantDashDot", "color": "0000FF"}}},
+        ),
         # ALIGNMENT
         # - horizontal
         ("text-align: center", {"alignment": {"horizontal": "center"}}),
@@ -249,6 +261,20 @@ def test_css_to_excel_multiple():
     } == actual
 
 
+@pytest.mark.parametrize(
+    "css",
+    [
+        "border-top-style: unhandled-border-style",
+        "border-style: another-unhandled-style",
+    ],
+)
+def test_css_to_excel_unhandled_border_style_warns(css):
+    """Test that unhandled border styles raise a CSSWarning."""
+    convert = CSSToExcelConverter()
+    with tm.assert_produces_warning(CSSWarning, match="Unhandled border style format"):
+        convert(css)
+
+
 @pytest.mark.parametrize(
     "css,inherited,expected",
     [
@@ -330,6 +356,23 @@ def test_css_to_excel_bad_colors(input_color):
         assert expected == convert(css)
 
 
+@pytest.mark.parametrize("input_color", ["#", "#1234567"])
+def test_css_to_excel_invalid_color_raises(input_color):
+    """Test that invalid colors raise a ValueError."""
+    css = (
+        f"border-top-color: {input_color}; "
+        f"border-right-color: {input_color}; "
+        f"border-bottom-color: {input_color}; "
+        f"border-left-color: {input_color}; "
+        f"background-color: {input_color}; "
+        f"color: {input_color}"
+    )
+
+    convert = CSSToExcelConverter()
+    with pytest.raises(ValueError, match=f"Unexpected color {input_color}"):
+        convert(css)
+
+
 def tests_css_named_colors_valid():
     upper_hexs = set(map(str.upper, string.hexdigits))
     for color in CSSToExcelConverter.NAMED_COLORS.values():