Fix Excel-specific border styles

doc/source/user_guide/style.ipynb

@@ -1594,8 +1594,9 @@
     "\n",
     "\n",
     "- Only CSS2 named colors and hex colors of the form `#rgb` or `#rrggbb` are currently supported.\n",
-    "- The following pseudo CSS properties are also available to set excel specific style properties:\n",
+    "- The following pseudo CSS properties are also available to set Excel specific style properties:\n",
     "    - `number-format`\n",
+    "    - `border-style` (for Excel-specific styles: \"hair\", \"mediumDashDot\", \"dashDotDot\", \"mediumDashDotDot\", \"dashDot\", \"slandDashDot\", or \"mediumDashed\")\n",
     "\n",
     "Table level styles, and data cell CSS-classes are not included in the export to Excel: individual cells must have their properties mapped by the `Styler.apply` and/or `Styler.applymap` methods."
    ]

doc/source/whatsnew/v1.5.1.rst

@@ -90,6 +90,7 @@ Bug fixes
 - Bug in :func:`assert_index_equal` for extension arrays with non matching ``NA`` raising ``ValueError`` (:issue:`48608`)
 - Bug in :meth:`DataFrame.pivot_table` raising unexpected ``FutureWarning`` when setting datetime column as index (:issue:`48683`)
 - Bug in :meth:`DataFrame.sort_values` emitting unnecessary ``FutureWarning`` when called on :class:`DataFrame` with boolean sparse columns (:issue:`48784`)
+- Bug in :meth:`.Styler.to_excel` leading to error when unrecognized ``border-style`` (including ``"hair"``) provided to Excel writers (:issue:`48649`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/io/formats/excel.py

@@ -299,6 +299,24 @@ def _border_style(self, style: str | None, width: str | None, color: str | None)
             if width_name in ("hair", "thin"):
                 return "dashed"
             return "mediumDashed"
+        elif style in (
+            "hair",
+            "mediumDashDot",
+            "dashDotDot",
+            "mediumDashDotDot",
+            "dashDot",
+            "slantDashDot",
+            "mediumDashed",
+        ):
+            # Excel-specific styles
+            return style
+        else:
+            warnings.warn(
+                f"Unhandled border style format: {repr(style)}",
+                CSSWarning,
+                stacklevel=find_stack_level(inspect.currentframe()),
+            )
+            return "none"
 
     def _get_width_name(self, width_input: str | None) -> str | None:
         width = self._width_to_float(width_input)

pandas/tests/io/excel/test_style.py

@@ -115,6 +115,12 @@ def test_styler_to_excel_unstyled(engine):
         ["border", "left", "color", "rgb"],
         {"xlsxwriter": "FF111222", "openpyxl": "00111222"},
     ),
+    # Border styles
+    (
+        "border-left-style: hair; border-left-color: black",
+        ["border", "left", "style"],
+        "hair",
+    ),
 ]