diff --git a/nibabel/gifti/gifti.py b/nibabel/gifti/gifti.py
index 56efa4ea0..16261ee67 100644
--- a/nibabel/gifti/gifti.py
+++ b/nibabel/gifti/gifti.py
@@ -852,7 +852,7 @@ def _to_xml_element(self):
             GIFTI.append(dar._to_xml_element())
         return GIFTI
 
-    def to_xml(self, enc='utf-8', *, mode='strict') -> bytes:
+    def to_xml(self, enc='utf-8', *, mode='strict', **kwargs) -> bytes:
         """Return XML corresponding to image content"""
         if mode == 'strict':
             if any(arr.datatype not in GIFTI_DTYPES for arr in self.darrays):
@@ -882,7 +882,7 @@ def to_xml(self, enc='utf-8', *, mode='strict') -> bytes:
         header = b"""<?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE GIFTI SYSTEM "http://www.nitrc.org/frs/download.php/115/gifti.dtd">
 """
-        return header + super().to_xml(enc)
+        return header + super().to_xml(enc, **kwargs)
 
     # Avoid the indirection of going through to_file_map
     def to_bytes(self, enc='utf-8', *, mode='strict'):
diff --git a/nibabel/xmlutils.py b/nibabel/xmlutils.py
index 31637b5e0..4a5fb2897 100644
--- a/nibabel/xmlutils.py
+++ b/nibabel/xmlutils.py
@@ -15,17 +15,24 @@
 
 
 class XmlSerializable:
-    """Basic interface for serializing an object to xml"""
+    """Basic interface for serializing an object to XML"""
 
-    def _to_xml_element(self):
+    def _to_xml_element(self) -> Element:
         """Output should be a xml.etree.ElementTree.Element"""
-        raise NotImplementedError()
+        raise NotImplementedError  # pragma: no cover
 
-    def to_xml(self, enc='utf-8'):
-        """Output should be an xml string with the given encoding.
-        (default: utf-8)"""
+    def to_xml(self, enc='utf-8', **kwargs) -> bytes:
+        r"""Generate an XML bytestring with a given encoding.
+
+        Parameters
+        ----------
+        enc : :class:`string`
+            Encoding to use for the generated bytestring. Default: 'utf-8'
+        \*\*kwargs : :class:`dict`
+            Additional keyword arguments to :func:`xml.etree.ElementTree.tostring`.
+        """
         ele = self._to_xml_element()
-        return '' if ele is None else tostring(ele, enc)
+        return b'' if ele is None else tostring(ele, enc, **kwargs)
 
 
 class XmlBasedHeader(FileBasedHeader, XmlSerializable):
@@ -101,10 +108,10 @@ def parse(self, string=None, fname=None, fptr=None):
         parser.ParseFile(fptr)
 
     def StartElementHandler(self, name, attrs):
-        raise NotImplementedError
+        raise NotImplementedError  # pragma: no cover
 
     def EndElementHandler(self, name):
-        raise NotImplementedError
+        raise NotImplementedError  # pragma: no cover
 
     def CharacterDataHandler(self, data):
-        raise NotImplementedError
+        raise NotImplementedError  # pragma: no cover